direnv: update for new nushell behavior (#5880)

Nushell 0.98 makes it so external commands returning non-zero
is considered an exception[0]. Wrap the direnv invocation in a
try-catch, so that even if direnv fails, we get the message
from direnv, but not the traceback from nushell.

[0]: https://www.nushell.sh/blog/2024-09-17-nushell_0_98_0.html#non-zero-exit-codes-are-now-errors-toc

tests/modules/programs/direnv/nushell.nix

@@ -14,6 +14,6 @@
   in ''
     assertFileExists "${configFile}"
     assertFileRegex "${configFile}" \
-      '^\s*let direnv = (/nix/store/.*direnv.*/bin/direnv export json$'
+      '^\s*/nix/store/.*direnv.*/bin/direnv export json$'
   '';
 }