build: bundle-libraries.sh: do not override argv[0] in inner exec calls

Only mangle argv[0] of the first executed process and leave the argument
vector of subsequent invocations as-is to allow child programs to properly
discover resources relative to their binary locations.

Fixes "cc1" discovery when executing the host gcc through the bundled
"ccache" executable.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 9e64874fb2)

scripts/bundle-libraries.sh

@@ -79,8 +79,10 @@ _runas_so() {
 		int mangle_arg0(int argc, char **argv, char **env) {
 			char *arg0 = getenv("RUNAS_ARG0");
 
-			if (arg0)
+			if (arg0) {
 				argv[0] = arg0;
+				unsetenv("RUNAS_ARG0");
+			}
 
 			return 0;
 		}