xsession: remove bashisms in start scripts

Fixes #836

(cherry picked from commit 0dfa1eef25)

modules/xsession.nix

@@ -110,7 +110,7 @@ in
     home.file.".xprofile".text = ''
         . "${config.home.profileDirectory}/etc/profile.d/hm-session-vars.sh"
 
-        if [[ -e "$HOME/.profile" ]]; then
+        if [ -e "$HOME/.profile" ]; then
           . "$HOME/.profile"
         fi
 
@@ -135,7 +135,7 @@ in
     home.file.${cfg.scriptPath} = {
       executable = true;
       text = ''
-        if [[ ! -v HM_XPROFILE_SOURCED ]]; then
+        if [ -z "$HM_XPROFILE_SOURCED" ]; then
           . ~/.xprofile
         fi
         unset HM_XPROFILE_SOURCED
@@ -150,7 +150,7 @@ in
         systemctl --user stop graphical-session-pre.target
 
         # Wait until the units actually stop.
-        while [[ -n "$(systemctl --user --no-legend --state=deactivating list-units)" ]]; do
+        while [ -n "$(systemctl --user --no-legend --state=deactivating list-units)" ]; do
           sleep 0.5
         done
       '';