Fix a few issues that I ran into when running make cluster (#698)

I ran make cluster with
```
make cluster -j20 TMUX_LAYOUT=tiled NODES=3
```

Author: JelteF

Makefile

@@ -195,13 +195,13 @@ $(TMUX_SCRIPT): bin
 
 tmux-script: $(TMUX_SCRIPT) ;
 
-tmux-clean:
+tmux-clean: bin
 	$(PG_AUTOCTL) do tmux clean           \
          --root $(TMUX_TOP_DIR)           \
          --first-pgport $(FIRST_PGPORT)   \
          --nodes $(NODES)
 
-tmux-session:
+tmux-session: bin
 	$(PG_AUTOCTL) do tmux session         \
          --root $(TMUX_TOP_DIR)           \
          --first-pgport $(FIRST_PGPORT)   \
@@ -213,7 +213,11 @@ tmux-session:
          --binpath $(BINPATH)             \
          --layout $(TMUX_LAYOUT)
 
-cluster: install tmux-clean tmux-session ;
+cluster: install tmux-clean
+	# This is explicitly not a target, otherwise when make uses multiple jobs
+	# tmux-clean and tmux-session can have a race condidition where tmux-clean
+	# removes the files that are just created by tmux-session.
+	$(MAKE) tmux-session
 
 valgrind-session: build-test
 	docker run                             \

src/bin/pg_autoctl/main.c

@@ -130,14 +130,19 @@ main(int argc, char **argv)
 	 * environment variable.
 	 */
 	strlcpy(pg_autoctl_argv0, argv[0], MAXPGPATH);
-	if (!get_env_copy("PG_AUTOCTL_DEBUG_BIN_PATH", pg_autoctl_program, MAXPGPATH))
+	if (env_exists("PG_AUTOCTL_DEBUG_BIN_PATH"))
 	{
-		if (!set_program_absolute_path(pg_autoctl_program, MAXPGPATH))
+		if (!get_env_copy("PG_AUTOCTL_DEBUG_BIN_PATH", pg_autoctl_program, MAXPGPATH))
 		{
 			/* errors have already been logged */
 			exit(EXIT_CODE_INTERNAL_ERROR);
 		}
 	}
+	else if (!set_program_absolute_path(pg_autoctl_program, MAXPGPATH))
+	{
+		/* errors have already been logged */
+		exit(EXIT_CODE_INTERNAL_ERROR);
+	}
 
 	if (!commandline_run(&command, argc, argv))
 	{