ci: bootstrap from the index when validating an unreleased mcpp

The pin may name the very release the run is validating, which does not
exist yet — that is what MCPP_SOURCE_REF is for. Bootstrap from whatever
the index has; the build under review replaces it a step later.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -74,7 +74,15 @@ jobs:
       - name: Install mcpp
         run: |
           xlings update
-          xlings install "mcpp@$MCPP_VERSION" -y -g
+          # ⚠️ THE PIN MAY NAME THE RELEASE THIS RUN IS VALIDATING, which does
+          # not exist yet — that is the whole point of MCPP_SOURCE_REF. Bootstrap
+          # from whatever the index has; the step below replaces it with the
+          # build under review, and the pin is what an ordinary run tests.
+          if [ -n "${MCPP_SOURCE_REF:-}" ]; then
+            xlings install mcpp -y -g
+          else
+            xlings install "mcpp@$MCPP_VERSION" -y -g
+          fi
           mcpp --version
           mcpp self config --mirror GLOBAL
           # ⭐⭐ CROSS-VALIDATION: BUILD THE mcpp UNDER REVIEW AND USE THAT ONE.