tsweb/varz: add binary name to version metric

Fixes tailscale/corp#29530

Change-Id: Iae04456d7ac5527897f060370e90c9517c00a818
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f