Use alpine pwsh images in TestPwshGitCredHelper

Backport of !7307 (merged) to 19-3-stable.

Debian 11 (bullseye) left LTS on 2026-08-31 and its security repository was removed from security.debian.org. In the *-debian-11 pwsh images, apt-get install git now 404s, and TestPwshGitCredHelper fails deterministically.

Switches the linux images used by TestPwshGitCredHelper from the debian-11 variants of mcr.microsoft.com/powershell to the alpine variants of the same pwsh versions, installing git with apk add instead of apt-get. Already merged to main (and present on 19-4-stable since that branch was cut afterward) — this MR ports the same commit onto 19-3-stable.