bashrc: move call for $nixos_needsreboot into `PS0` to prevent setting `$?` to `0`

.bashrc

@@ -219,8 +219,11 @@ function nixos_needsreboot() {
 }
 
 # PS1 setup
-PS0="\t\n" # display time in HH:MM:SS format
-PS1='$(nixos_needsreboot)'"\n[\u@\h:\$PWD \$?]\$ "
+PS0="\t" # display time in HH:MM:SS format
+# the function needs to be called **everytime** and is in single quotes
+# shellcheck disable=SC2016
+PS0+='$(nixos_needsreboot)'"\n"
+PS1="\n[\u@\h:\$PWD \$?]\$ "
 export PS0 PS1
 
 # zoxide setup