git-fetch-with-cli: Set GIT_DIR for bare repository compatibility (#14860)

When using `net.git-fetch-with-cli = true` , Cargo fails to clone the
git repository into the cache because `safe.bareRepository` defaults to
`explicit`. This results in an error stating that a bare repository
cannot be used.

This patch sets the `GIT_DIR` environment variable to the correct
repository path instead of removing it. This ensures that Git uses the
correct repository directory and avoids the "cannot use bare repository"
error.

Fixes #14758

Author: weihanglo

src/cargo/sources/git/utils.rs

@@ -1101,9 +1101,8 @@ fn fetch_with_cli(
         .args(refspecs)
         // If cargo is run by git (for example, the `exec` command in `git
         // rebase`), the GIT_DIR is set by git and will point to the wrong
-        // location (this takes precedence over the cwd). Make sure this is
-        // unset so git will look at cwd for the repo.
-        .env_remove("GIT_DIR")
+        // location. This makes sure GIT_DIR is always the repository path.
+        .env("GIT_DIR", repo.path())
         // The reset of these may not be necessary, but I'm including them
         // just to be extra paranoid and avoid any issues.
         .env_remove("GIT_WORK_TREE")