Always downgrade the registry protocol to supported versions

Not all version support the new sparse protocol. While old versions
ignore the value, 1.66 and other fail due to unstable features.

If such a version is detected, always downgrade to the git protocol.

This fixes running the action twice with different toolchains. Even if
the first install uses something which supports "sparse", the second run
can still downgrade it to "git".

Closes #12

Author: jonasbb

action.yml

@@ -85,6 +85,9 @@ runs:
         if [[ ! -v CARGO_UNSTABLE_SPARSE_REGISTRY ]]; then
           echo "CARGO_UNSTABLE_SPARSE_REGISTRY=true" >> $GITHUB_ENV
         fi
+        if [[ ! -v CARGO_REGISTRIES_CRATES_IO_PROTOCOL ]]; then
+          echo "CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse" >> $GITHUB_ENV
+        fi
       shell: bash
     - name: "Install Rust Problem Matcher"
       run: echo "::add-matcher::${{ github.action_path }}/rust.json"
@@ -126,17 +129,15 @@ runs:
         echo "cachekey=$(echo $DATE$HASH | head -c12)" >> $GITHUB_OUTPUT
       shell: bash
 
-    # Copied from dtolnay/rust-toolchain and adapted
-    # https://github.com/dtolnay/rust-toolchain/blob/25dc93b901a87e864900a8aec6c12e9aa794c0c3/action.yml#L100-L108
-    - name: "Enable cargo sparse registry on stable"
+    - name: "Downgrade registry access protocol when needed"
       run: |
-        # except on 1.66 and 1.67, on which it is unstable
-        # Not all 1.68.0-nightly versions support it either
+        # Not all versions support setting CARGO_REGISTRIES_CRATES_IO_PROTOCOL
+        # On versions 1.66, 1.67, and 1.68.0-nightly the value "sparse" is still unstable.
         # https://github.com/dtolnay/rust-toolchain/pull/69#discussion_r1107268108
-        if [[ ! -v CARGO_REGISTRIES_CRATES_IO_PROTOCOL ]]; then
-          if echo "${{steps.versions.outputs.rustc-version}}" | grep --invert --quiet '^rustc \(1\.6[67]\.\|1\.68\.0-nightly\)'; then
-            echo "CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse" >> $GITHUB_ENV
-          fi
+        # If we detect an incompatible value, set it to "git" which is always supported.
+        if [[ "${{steps.versions.outputs.rustc-version}}" =~ ^rustc\ (1\.6[67]\.|1\.68\.0-nightly) && "${CARGO_REGISTRIES_CRATES_IO_PROTOCOL}" == "sparse" ]]; then
+          echo "Downgrade cargo registry protocol to git"
+          echo "CARGO_REGISTRIES_CRATES_IO_PROTOCOL=git" >> $GITHUB_ENV
         fi
       shell: bash