Fix build failure when git is not in `/usr/bin`
Fix build failure when git is not in `/usr/bin`

Top commit date command is currently resetting its environment to just
`TZ`, which, at least on CentOS, means that the `git` binary cannot be
found, because it's installed into `/usr/local/bin` in the build
container.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb