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MINOR: [Go][CI][Benchmarking] Fix GitHub info in Go benchmarks (#14952)

This PR changes `go_bench_adapt.py` to be more accurate about reporting GitHub information to Conbench from a local computer.

In most cases, this script will be run on the master branch during [this workflow](https://github.com/apache/arrow/blob/794aefd6409a3da339ecc1a3d1ea6466b3d9ddb6/.github/workflows/go.yml#L94). This PR shouldn't change that use case. 

However, rarely, someone might [run benchmarks locally](https://conbench.ursa.dev/?search=branch) on a branch on their computer. If that's the case, we want the GitHub information to be scraped from the local repo, instead of assuming it's on the master branch. This is accomplished by passing `None` to `github`.

Authored-by: Austin Dickey <austin3spammy@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>