Speed up the Build workflow

- Pass the MariaDB client library to mysqlclient-sys through its environment variables, so a cached build no longer recompiles mysqlclient-sys, diesel and diesel_migrations in every MySQL build. The cache prefix changes once, because the old caches would keep rebuilding it.
- Drop the sqlite,mysql,postgresql,enable_mimalloc test: enable_mimalloc is still tested on and off.
- Only save caches on main, so pull requests and branches no longer push main's caches out of the 10 GB limit.

.github/workflows/build.yml

@@ -57,7 +57,25 @@ jobs:
     steps:
       # Install dependencies
       - name: "Install dependencies Ubuntu"
-        run: sudo apt-get update && sudo apt-get install -y --no-install-recommends openssl build-essential libmariadb-dev-compat libpq-dev libssl-dev pkg-config
+        run: |
+          # Separate commands, so a failing update fails the step. The image already has Oracle's MySQL client
+          # library, so a skipped install would otherwise go unnoticed and build against that one.
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends openssl build-essential libmariadb-dev-compat libpq-dev libssl-dev pkg-config
+
+          # Pass the MariaDB client library to mysqlclient-sys through its environment variables.
+          # When it finds the library with pkg-config, its build script makes cargo rerun it whenever
+          # anything in the library directory changes, and the install above changes that directory on
+          # every run. So every MySQL build recompiled mysqlclient-sys, diesel and diesel_migrations,
+          # even with a cached build. Cargo compares environment variables by their value instead.
+          lib_dir="$(pkg-config --variable=libdir mysqlclient)"
+          version="$(pkg-config --modversion mysqlclient)"
+          {
+            echo "MYSQLCLIENT_NO_PKG_CONFIG=1"
+            echo "LIBMARIADB_NO_PKG_CONFIG=1"
+            echo "MYSQLCLIENT_LIB_DIR=${lib_dir}"
+            echo "MYSQLCLIENT_VERSION=${version}"
+          } | tee -a "${GITHUB_ENV}"
       # End Install dependencies
 
       # Checkout the repo
@@ -118,23 +136,21 @@ jobs:
           # Use a custom prefix-key to force a fresh start. This is sometimes needed with bigger changes.
           # Like changing the build host from Ubuntu 20.04 to 22.04 for example.
           # Only update when really needed! Use a <year>.<month>[.<inc>] format.
-          prefix-key: "v2025.09-rust"
+          prefix-key: "v2026.11-rust"
+          # Only main saves caches. The repository can keep 10 GB of caches, and the ones saved by pull
+          # requests and other branches were mostly copies of main's that pushed other caches out.
+          save-if: ${{ github.ref == 'refs/heads/main' }}
       # End Enable Rust Caching
 
       # Run cargo tests
       # First test all features together, afterwards test them separately.
+      # enable_mimalloc only selects the allocator, so it is tested on and off, not with every combination.
       - name: "test features: sqlite,mysql,postgresql,enable_mimalloc,s3"
         id: test_sqlite_mysql_postgresql_mimalloc_s3
         if: ${{ !cancelled() }}
         run: |
           cargo test --profile ci --features sqlite,mysql,postgresql,enable_mimalloc,s3
 
-      - name: "test features: sqlite,mysql,postgresql,enable_mimalloc"
-        id: test_sqlite_mysql_postgresql_mimalloc
-        if: ${{ !cancelled() }}
-        run: |
-          cargo test --profile ci --features sqlite,mysql,postgresql,enable_mimalloc
-
       - name: "test features: sqlite,mysql,postgresql"
         id: test_sqlite_mysql_postgresql
         if: ${{ !cancelled() }}
@@ -198,7 +214,6 @@ jobs:
         if: ${{ failure() }}
         env:
           TEST_DB_M_S3: ${{ steps.test_sqlite_mysql_postgresql_mimalloc_s3.outcome }}
-          TEST_DB_M: ${{ steps.test_sqlite_mysql_postgresql_mimalloc.outcome }}
           TEST_DB: ${{ steps.test_sqlite_mysql_postgresql.outcome }}
           TEST_SQLITE: ${{ steps.test_sqlite.outcome }}
           TEST_MYSQL: ${{ steps.test_mysql.outcome }}
@@ -213,7 +228,6 @@ jobs:
           echo "|Job|Status|" >> "${GITHUB_STEP_SUMMARY}"
           echo "|---|------|" >> "${GITHUB_STEP_SUMMARY}"
           echo "|test (sqlite,mysql,postgresql,enable_mimalloc,s3)|${TEST_DB_M_S3}|" >> "${GITHUB_STEP_SUMMARY}"
-          echo "|test (sqlite,mysql,postgresql,enable_mimalloc)|${TEST_DB_M}|" >> "${GITHUB_STEP_SUMMARY}"
           echo "|test (sqlite,mysql,postgresql)|${TEST_DB}|" >> "${GITHUB_STEP_SUMMARY}"
           echo "|test (sqlite)|${TEST_SQLITE}|" >> "${GITHUB_STEP_SUMMARY}"
           echo "|test (mysql)|${TEST_MYSQL}|" >> "${GITHUB_STEP_SUMMARY}"