.github/workflows/ci.yml: Pin compiler and OS versions

Avoid randomly-starting failures which disrupt established processes.

Author: CyberShadow

.github/workflows/ci.yml

@@ -7,8 +7,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest ]
-        dc: [ dmd-latest, dmd-beta, ldc-latest ]
+        os: [ ubuntu-20.04 ]
+        dc: [ dmd-2.100.2, ldc-1.30.0 ]
 
     runs-on: ${{ matrix.os }}
     steps: