Use same BuildDate format as upstream

PKGBUILD

@@ -28,7 +28,7 @@ build() {
       -X github.com/prometheus/common/version.Revision=$pkgver \
       -X github.com/prometheus/common/version.Branch=tarball \
       -X github.com/prometheus/common/version.BuildUser=someone@builder \
-      -X github.com/prometheus/common/version.BuildDate=$SOURCE_DATE_EPOCH" \
+      -X github.com/prometheus/common/version.BuildDate=$(date -d@"$SOURCE_DATE_EPOCH" +%Y%m%d-%H:%M:%S)" \
     .
 }