fixup! release: create initial Windows installer build workflow

Loosen the prereqs step enough to allow "-<something>" pre-release tags
(e.g. "-rc0"). Also switch to 'grep -E' to avoid needing to escape so many
things.

.github/workflows/build-git-installers.yml

@@ -17,7 +17,7 @@ jobs:
       - name: Validate tag
         run: |
           echo "$GITHUB_REF" |
-          grep '^refs/tags/v2\.\(0\|[1-9][0-9]*\)\.\(0\|[1-9][0-9]*\)\.vfs\.0\.\(0\|[1-9][0-9]*\)$' || {
+          grep -E '^refs/tags/v2\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.vfs\.0\.(0|[1-9][0-9]*)(-[a-z0-9]+)?$' || {
             echo "::error::${GITHUB_REF#refs/tags/} is not of the form v2.<X>.<Y>.vfs.0.<W>" >&2
             exit 1
           }