gh/get-etag.sh: sanitize ETAGs

XIEMA uses something like (for ARM mac):
"cs-cz|6ea47896-cfc1-4c33-a2cc-5dc7ee38de39|15.05.2024 11:57:53|LiveSite"

where the spaces and pipes may cause problems later if not properly
sanitized. Doesn't seem to cause problems so far but perhaps still better
to sanitize.

Keep just chars from Portable Filename Character Set (alnums + <space>,
<dash-minus>, <underscore>), other replace with _.

Author: MartinPulec

.github/scripts/get-etag.sh

@@ -4,7 +4,7 @@
 
 ETAG=$(curl -LI "$1" | grep -i '^etag' | sed 's/.*"\(.*\)".*/\1/')
 if [ "$ETAG" ]; then
-        printf '%s\n' "$ETAG"
+        printf '%s\n' "$ETAG" | sed 's/[^-._A-Za-z0-9]/_/g'
         exit 0
 fi