swayidle: fix systemd service

swayidle executes commands using "sh -c" and so its PATH must contain
a shell. This adds such PATH entry to the environment of the systemd
service.

Fixes #2811.

tests/modules/services/swayidle/basic-configuration.nix

@@ -47,6 +47,9 @@
 
       assertFileExists $serviceFile
       assertFileRegex $serviceFile 'ExecStart=.*/bin/swayidle ${expectedArgs}'
+      assertFileRegex $serviceFile 'Environment=.*PATH=${
+        lib.makeBinPath [ pkgs.bash ]
+      }'
     '';
   };
 }