Fix toolchain workflow to ensure use of intended toolchain

.github/workflows/ci-toolchain.yml

@@ -46,6 +46,12 @@ jobs:
 
     # We can start using the alr we just built
 
+    - name: Select toolchain GNAT^${{matrix.gcc_version}}
+      run: ./bin/alr -d -n toolchain --select gnat_native^${{matrix.gcc_version}} gprbuild
+
+    - name: Show builder alr configuration
+      run: ./bin/alr -d -n version
+
     - name: Update dependencies
       run: ./bin/alr -d -n update
 
@@ -67,6 +73,10 @@ jobs:
     - name: Show built version
       run: ./bin/alr -d -n version
 
+    - name: Verify proper toolchain used for build
+      shell: bash
+      run: ./bin/alr -d -n printenv | grep 'compiled with version' | grep -q '${{ matrix.gcc_version }}'
+
     # Run the testsuite with the just build alr. The testsuite picks the proper
     # alr in the ./bin/alr location.