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

modules/programs/direnv.nix

@@ -140,9 +140,15 @@ in {
             $env.config.hooks.pre_prompt?
             | default []
             | append {||
-                let direnv = (${getExe cfg.package} export json
+                let direnv = (
-                | from json
+                    try {
-                | default {})
+                        ${getExe cfg.package} export json
+                    } catch {
+                        ""
+                    }
+                    | from json
+                    | default {}
+                )
                 if ($direnv | is-empty) {
                     return
                 }

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$'
+      '^\s*/nix/store/.*direnv.*/bin/direnv export json$'
   '';
 }