Add CTAGS_COMMAND to zoekt indexer environment variables

What does this MR do?

Configures zoekt to use universal ctags in staging. I verified that /usr/bin/ctags is indeed universal ctags.

➜  gitlab-build-images git:(jm-add-zoekt-ctags) docker run --rm -it registry.gitlab.com/gitlab-org/build/cng/gitlab-zoekt-indexer:v16.9.1 /bin/sh
$ type ctags
ctags is /usr/bin/ctags
$ ctags --version
Universal Ctags 5.9.0, Copyright (C) 2015 Universal Ctags Team
Universal Ctags is derived from Exuberant Ctags.
Exuberant Ctags 5.8, Copyright (C) 1996-2009 Darren Hiebert
  Compiled: Sep  3 2021, 18:12:18
  URL: https://ctags.io/
  Optional compiled features: +wildcards, +regex, +gnulib_regex, +iconv, +option-directory, +xpath, +json, +interactive, +sandbox, +yaml, +packcc, +optscript
$

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