[FEAT] Conditionally link devenv profile

When loading a shell, devenv checks .devenv/profile is

1. not a link
2. readlink reports someplace other than ${profile}

Then will remove whatever is .devenv/profile and relink to the current
active profile. When running a single command at a time, this is
usually not a problem, but if you attempt to run two commands at the
same time, then the race condition of one process removing a link while
another is creating the link will emit warnings to stderr.

src/modules/top-level.nix

@@ -190,8 +190,11 @@ in
       fi
 
       mkdir -p .devenv
-      rm -f .devenv/profile
-      ln -s ${profile} .devenv/profile
+      if [ ! -L .devenv/profile ] || [ "$(${pkgs.coreutils}/bin/readlink .devenv/profile)" != "${profile}" ]
+      then
+        rm -f .devenv/profile
+        ln -nsf ${profile} .devenv/profile
+      fi
     '';
 
     shell = performAssertions (