Build: remove -a from build to speed up rebuilds (#13026)

I think this is a hold-over from when Go was less careful about separating architectures.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

.promu.yml

@@ -18,7 +18,6 @@ build:
         windows:
             - builtinassets
             - stringlabels
-    flags: -a
     ldflags: |
         -X github.com/prometheus/common/version.Version={{.Version}}
         -X github.com/prometheus/common/version.Revision={{.Revision}}