home-environment: prepend to PATH in sessionPath

This makes the options behave as people expect it to: one's personal
configuration can shadow system executables.

Closes #3324.

modules/home-environment.nix

@@ -556,7 +556,7 @@ in
 
         ${config.lib.shell.exportAll cfg.sessionVariables}
       '' + lib.optionalString (cfg.sessionPath != [ ]) ''
-        export PATH="$PATH''${PATH:+:}${concatStringsSep ":" cfg.sessionPath}"
+        export PATH="${concatStringsSep ":" cfg.sessionPath}''${PATH:+:}$PATH"
       '' + cfg.sessionVariablesExtra;
     };
 

tests/modules/home-environment/session-path.nix

@@ -10,6 +10,6 @@
     hmSessVars=home-path/etc/profile.d/hm-session-vars.sh
     assertFileExists $hmSessVars
     assertFileContains $hmSessVars \
-      'export PATH="$PATH''${PATH:+:}bar:baz:foo"'
+      'export PATH="bar:baz:foo''${PATH:+:}$PATH"'
   '';
 }