fix(go-runner): use correct branch on main

[not-matthias]

Used a slightly different logic than in the runner, but verified with other repos using a similar approach:

• https://github.com/ethereum-optimism/op-geth/blob/7beb36e65e581040578092f61f810540390a60dd/internal/build/env.go#L98-L108
• https://github.com/porter-dev/porter-archive/blob/ed607ee7d3ca765c65eda23987eed6a41502f09f/cli/cmd/v2/apply.go#L367C17-L373

GITHUB_HEAD_REF is only set for the PR, we can fallback to GITHUB_REF_NAME in any other case.

[codspeed-hq]

CodSpeed Performance Report

Merging #15 will degrade performances by 16.4%

_{Comparing cod-1321-fix-incorrect-version-selection-in-codspeed-go (c396b8e) with main (33bf04e)¹}

Summary

❌ 2 regressions
✅ 16 untouched
🆕 4 new

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	BenchmarkQuicktest	N/A	179.3 µs	N/A
🆕	BenchmarkWithSlogAssert	N/A	260.8 µs	N/A
🆕	BenchmarkTestifyWithNew	N/A	113 µs	N/A
🆕	BenchmarkTestifyWithT	N/A	85.1 µs	N/A
❌	BenchmarkFibonacci20_Loop	110.4 µs	132 µs	-16.4%
❌	BenchmarkSleep1us_Loop	1.8 µs	2 µs	-10.43%

Footnotes

1. No successful run was found on main (ed306a6) during the generation of this report, so 33bf04e was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[art049]

lgtm!