GitHub Actions: Relax the regex to match the generix-unix archive

[Why]
Our alpha packages published to `rabbitmq/server-packages` use letters
in addition to digits and dots.

[How]
Accept any string that starts with a digit as the version here. We just
enforce the prefix and the filename extension.

The digit at the beginning it here to exclude the
`rabbitmq-server-generic-unix-latest-toolchain-*.tar.xz` archive.

Author: dumbbell

.github/workflows/test-make-target.yaml

@@ -59,7 +59,7 @@ jobs:
       with:
         version: 'tags/v4.0.3'
         regex: true
-        file: "rabbitmq-server-generic-unix-[\\d.]*\\.tar\\.xz"
+        file: "rabbitmq-server-generic-unix-\\d.+\\.tar\\.xz"
         target: ./
 
     - name: MIXED CLUSTERS - SETUP SECONDARY_DIST