cmd/tailscale/cli: omit "file" subcommand if taildrop is omitted from build

Updates #15812
Updates #12614

Change-Id: Ic945b26a127ba15399abdaab8fe43b1cfa64d874
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f