gamemodelist: Fix unreadable process maps

Systemd commit [bf1b9ae487b65b1cb1639b222724fab95e508cf5](https://github.com/systemd/systemd/commit/bf1b9ae487b65b1cb1639b222724fab95e508cf5) (present in Systemd v254 and later) effectively broke gamemodelist as the process map for the `systemd --user` process became unreadable. After this change gamemodelist would exit with an error like the following:

```
awk: fatal: cannot open file `/proc/2281/maps' for reading: Permission denied
```

To work around this let's add a hook to the awk statement to skip any files that can't be read.

Closes FeralInteractive/gamemode#456

data/gamemodelist

@@ -28,5 +28,5 @@ if [ ! -d /proc ]; then
 fi
 
 find /proc -maxdepth 2 -type f -user "${USER}" -readable -name maps -exec \
-  awk -- '$0 ~ /libgamemodeauto\.so\.0/ {pid=FILENAME; gsub("[^0-9]", "", pid); print pid;nextfile}' {} + \
+  awk -- 'BEGINFILE { if (ERRNO) nextfile } $0 ~ /libgamemodeauto\.so\.0/ {pid=FILENAME; gsub("[^0-9]", "", pid); print pid;nextfile}' {} + \
   | xargs | xargs -I{} -- ps -o pid,ppid,user,ni,psr,comm --pid '{}'