system tests: use repository mirrors on S3 for reproducibility

no direct internet download from apt repositories,
which over time will change or cease to exist.

also migrate to gpg2 on newer ubuntu.

api/gpg.go

@@ -61,7 +61,7 @@ func apiGPGAddKey(c *gin.Context) {
 		args = append(args, keys...)
 	}
 
-	finder := pgp.GPG1Finder()
+	finder := pgp.GPGDefaultFinder()
 	gpg, _, err := finder.FindGPG()
 	if err != nil {
 		AbortWithJSONError(c, 400, err)