validate-install-from-source: enforce bullseye

Based on [1], it appears that .NET is not yet supported on Debian Bookworm
(12). As a result, attempting to install .NET in a container running Bookworm led to failures [2]. This change locks the validation workflow into using
Bullseye to work around this issue.

1: https://learn.microsoft.com/en-us/dotnet/core/install/linux-debian#supported-distributions
2: https://github.com/ldennington/git-credential-manager/actions/runs/5326250435/jobs/9648032190

Author: ldennington

.github/workflows/validate-install-from-source.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         vector:
         - image: ubuntu
-        - image: debian
+        - image: debian:bullseye
         - image: fedora
         # Centos no longer officially maintains images on Docker Hub. However,
         # tgagor is a contributor who pushes updated images weekly, which should