zsh: Complete --unsetenv like env -u

Signed-off-by: Simon McVittie <smcv@collabora.com>

completions/zsh/_bwrap

@@ -41,7 +41,7 @@ _bwrap_args=(
     '--sync-fd[Keep this fd open while sandbox is running]: :_guard "[0-9]#" "file descriptor to keep open"'
     '--tmpfs[Mount new tmpfs on DEST]:mount point for tmpfs:_files -/'
     '--uid[Custom uid in the sandbox (requires --unshare-user or --userns)]: :_guard "[0-9]#" "numeric group ID"'
-    '--unsetenv[Unset an environment variable]: :'
+    '--unsetenv[Unset an environment variable]:variable to unset:_parameters -g "*export*"'
     '--unshare-all[Unshare every namespace we support by default]'
     '--unshare-cgroup-try[Create new cgroup namespace if possible else continue by skipping it]'
     '--unshare-cgroup[Create new cgroup namespace]'