Run executables from PATH instead of /usr/bin

Not all distributions install non-system binaries into /usr/bin. For
example, NixOS installs packages to /nix/store using a unique hash
generated from the inputs used to build it:

/nix/store/jld7jh3ilvbg91zvn1bdyawfc55b9jk8-polkit-0.118-bin/bin/pkexec

daemon/gamemode-context.c

@@ -235,7 +235,7 @@ static int game_mode_set_governor(GameModeContext *self, enum GameModeGovernor g
 	}
 
 	const char *const exec_args[] = {
-		"/usr/bin/pkexec", LIBEXECDIR "/cpugovctl", "set", gov_str, NULL,
+		"pkexec", LIBEXECDIR "/cpugovctl", "set", gov_str, NULL,
 	};
 
 	LOG_MSG("Requesting update of governor policy to %s\n", gov_str);

daemon/gamemode-gpu.c

@@ -167,7 +167,7 @@ int game_mode_apply_gpu(const GameModeGPUInfo *info)
 
 	// Set up our command line to pass to gpuclockctl
 	const char *const exec_args[] = {
-		"/usr/bin/pkexec",
+		"pkexec",
 		LIBEXECDIR "/gpuclockctl",
 		device,
 		"set",

daemon/gamemode-tests.c

@@ -268,7 +268,7 @@ static int run_gamemoderun_and_reaper_tests(struct GameModeConfig *config)
 		/* Close stdout, we don't care if sh prints anything */
 		fclose(stdout);
 		/* Preload into sh and then kill it */
-		if (execl("/usr/bin/gamemoderun", "/usr/bin/gamemoderun", "sleep", "5", (char *)NULL) ==
+		if (execlp("gamemoderun", "gamemoderun", "sleep", "5", (char *)NULL) ==
 		    -1) {
 			LOG_ERROR("failed to launch gamemoderun with execl: %s\n", strerror(errno));
 			return -1;