Fix ui-post-install.sh to use the full username (#4809)

Fixes #4808 by extracting the full username by:

- Get PID using pgrep
- Get UID from PID using /proc/${PID}/loginuid
- Get user name from UID using id
Also replaces "complex" pipe from ps to sed with a (hopefully) "simpler" (as in requiring less knowledge about the arguments of ps and regexps) invocation of cat and id.

release_files/ui-post-install.sh

@@ -1,10 +1,15 @@
 #!/bin/sh
 
+set -e
+set -u
+
 # Check if netbird-ui is running
-if pgrep -x -f /usr/bin/netbird-ui >/dev/null 2>&1;
+pid="$(pgrep -x -f /usr/bin/netbird-ui || true)"
+if [ -n "${pid}" ]
 then
-  runner=$(ps --no-headers -o '%U' -p $(pgrep -x -f /usr/bin/netbird-ui) | sed 's/^[ \t]*//;s/[ \t]*$//')
+  uid="$(cat /proc/"${pid}"/loginuid)"
+  username="$(id -nu "${uid}")"
   # Only re-run if it was already running
   pkill -x -f /usr/bin/netbird-ui >/dev/null 2>&1
-  su -l - "$runner" -c 'nohup /usr/bin/netbird-ui > /dev/null 2>&1 &'
+  su - "${username}" -c 'nohup /usr/bin/netbird-ui > /dev/null 2>&1 &'
 fi