Support PowerShell 7

[markmssd]

Description

PowerShell 5.1 is the latest supported version, and does not receive any more updates. See https://learn.microsoft.com/en-us/powershell/scripting/install/powershell-support-lifecycle?view=powershell-7.5#windows-powershell-release-history. They now focus on the cross-platform version 7.

Difference between v5 and v7: https://learn.microsoft.com/en-us/powershell/scripting/whats-new/differences-from-windows-powershell?view=powershell-7.5#powershell-executable-changes

Buildkite Agent targets PowerShell 5.1 on Windows, which is convenient because it's always pre-installed on modern Windows. However, it'd be nice to configure it to use PowerShell 7 instead.

Context

Closes #3657

Changes

This PR does a few things to make it work:

• Look for .ps1 hooks even on non-Windows OS
• Add a new configuration BUILDKITE_HOOKS_SHELL / --hooks-shell, which works similar to BUILDKITE_SHELL/--shell, but for the Agent Hooks.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

[quinn-diesel]

hey @markmssd just wanted to jump in to confirm that we've got the PR on our list. we've been very swamped this week but we'll have a close look soon for you.