ci(buildkit): expose github runtime for gha tests

This exposes `ACTIONS_RUNTIME_TOKEN` and `ACTIONS_CACHE_URL`, which are
used to skip cache exporter tests, when combined with
https://github.com/moby/buildkit/commit/a8789cbd4a657836d1b747a9c4eb54fed4e221bd

Co-authored-by: Bjorn Neergaard <bjorn.neergaard@docker.com>
Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>
Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

.github/workflows/buildkit.yml

@@ -73,6 +73,12 @@ jobs:
             disabledFeatures="${disabledFeatures},merge_diff"
           fi
           echo "BUILDKIT_TEST_DISABLE_FEATURES=${disabledFeatures}" >> $GITHUB_ENV
+      # Expose `ACTIONS_RUNTIME_TOKEN` and `ACTIONS_CACHE_URL`, which is used
+      # in BuildKit's test suite to skip/unskip cache exporters:
+      # https://github.com/moby/buildkit/blob/567a99433ca23402d5e9b9f9124005d2e59b8861/client/client_test.go#L5407-L5411
+      -
+        name: Expose GitHub Runtime
+        uses: crazy-max/ghaction-github-runtime@v2
       -
         name: Checkout
         uses: actions/checkout@v3