test(index): enable SQLite index cache in CI

__CARGO_TEST_FORCE_SQLITE_INDEX_CACHE to force enable it.

.github/workflows/main.yml

@@ -185,6 +185,10 @@ jobs:
     - name: Clear test output
       run: ci/clean-test-output.sh
 
+    - name: Check operability of index cache in SQLite3
+      run: 'cargo test -p cargo --test testsuite -- alt_registry:: global_cache_tracker::'
+      env:
+        __CARGO_TEST_FORCE_SQLITE_INDEX_CACHE: 1
     # This only tests `cargo fix` because fix-proxy-mode is one of the most
     # complicated subprocess management in Cargo.
     - name: Check operability of rustc invocation with argfile

src/cargo/sources/registry/index/cache.rs

@@ -248,7 +248,10 @@ impl<'gctx> CacheManager<'gctx> {
     ///
     /// `root` --- The root path where caches are located.
     pub fn new(cache_root: Filesystem, gctx: &'gctx GlobalContext) -> CacheManager<'gctx> {
-        let store: Box<dyn CacheStore> = if gctx.cli_unstable().index_cache_sqlite {
+        #[allow(clippy::disallowed_methods)]
+        let use_sqlite = gctx.cli_unstable().index_cache_sqlite
+            || std::env::var("__CARGO_TEST_FORCE_SQLITE_INDEX_CACHE").is_ok();
+        let store: Box<dyn CacheStore> = if use_sqlite {
             Box::new(LocalDatabase::new(cache_root, gctx))
         } else {
             Box::new(LocalFileSystem::new(cache_root, gctx))