Make build script compatible with older gzip (prometheus#11256)

* Add RHEL7 compatibility for gzip options

Signed-off-by: Matthew Themis <matthew.themis@anatwine.com>
Signed-off-by: Matt <33697686+mttradebyte@users.noreply.github.com>
Co-authored-by: Julien Pivotto <roidelapluie@o11y.eu>

scripts/compress_assets.sh

@@ -6,6 +6,11 @@ set -euo pipefail
 
 cd web/ui
 cp embed.go.tmpl embed.go
+
+GZIP_OPTS="-fk"
+# gzip option '-k' may not always exist in the latest gzip available on different distros.
+if ! gzip -k -h &>/dev/null; then GZIP_OPTS="-f"; fi
+
 find static -type f -name '*.gz' -delete
-find static -type f -exec gzip -fk '{}' \; -print0 | xargs -0 -I % echo %.gz | xargs echo //go:embed >> embed.go
+find static -type f -exec gzip $GZIP_OPTS '{}' \; -print0 | xargs -0 -I % echo %.gz | xargs echo //go:embed >> embed.go
 echo var EmbedFS embed.FS >> embed.go