gpg-agent: don't abort nushell startup when tty is unavailable

The nushell integration set $env.GPG_TTY = (tty) unconditionally.
Unlike the bash/zsh/fish variants, which use a plain command
substitution and silently continue with an empty value on failure,
nushell aborts the whole script when an external command used as an
expression value exits non-zero. A missing controlling terminal (e.g.
a login shell invoked non-interactively, such as by WSL's generated
Windows app launchers) made nu exit immediately, breaking every
command run through that login shell.

Wrap the call in try/catch so nushell degrades the same way the other
shell integrations do.

modules/services/gpg-agent.nix

@@ -44,7 +44,7 @@ let
   '';
 
   gpgNushellInitStr = ''
-    $env.GPG_TTY = (tty)
+    $env.GPG_TTY = (try { tty } catch { "" })
   ''
   + optionalString cfg.enableSshSupport ''
     ${gpgSshSupportStr} | ignore

tests/modules/services/gpg-agent/ssh-support.nix

@@ -19,6 +19,9 @@
     assertFileContains \
       home-files/.config/nushell/config.nu \
       '$env.SSH_AUTH_SOCK = $"(@gnupg@/bin/gpgconf --list-dirs agent-ssh-socket)"'
+    assertFileContains \
+      home-files/.config/nushell/config.nu \
+      '$env.GPG_TTY = (try { tty } catch { "" })'
     assertFileContains \
       home-files/.zshenv \
       'export SSH_AUTH_SOCK="$(@gnupg@/bin/gpgconf --list-dirs agent-ssh-socket)"'