Confusing error on macOS `Second path fragment must not be a drive or UNC name`

Describe the bug
The runner is thinking in terms of Windows (.NET?) and yields an error message that is unintuitive to macOS users.

To Reproduce
Steps to reproduce the behavior:

1. Put this workflow into a repository: https://github.com/jsoref/macos-sip/blob/52ce3c8842c3787170acbd7ea5dfbde2e840c994/.github/workflows/test.yml
2. Trigger it.
3. See error

Expected behavior

1. Errors on macOS shouldn't talk about UNC paths since they really aren't a thing that macOS users encounter/use.
2. Report the value that's objectionable so that a human doesn't have to guess what the program saw.

Runner Version and Platform

Version of your runner?
2.291.1

OS of the machine running the runner? macOS 11.6.5 20G527

What's not working?

Error: Second path fragment must not be a drive or UNC name. (Parameter 'expression')

Job Log Output

2022-05-25T14:37:19.5557560Z Prepare workflow directory
2022-05-25T14:37:19.7519110Z Prepare all required actions
2022-05-25T14:37:20.2858270Z ##[group]Run brew install bash
2022-05-25T14:37:20.2858750Z brew install bash
2022-05-25T14:37:40.0739050Z shell: /bin/bash -e {0}
2022-05-25T14:37:40.0739540Z ##[endgroup]
2022-05-25T14:38:25.4328160Z ==> Downloading https://ghcr.io/v2/homebrew/core/bash/manifests/5.1.16
2022-05-25T14:38:29.2573490Z ==> Downloading https://ghcr.io/v2/homebrew/core/bash/blobs/sha256:4f387cc0993f868f31cd76483051a58420f80f57cf4626afc4b881d2a98959bb
2022-05-25T14:38:29.7039660Z ==> Downloading from https://pkg-containers.githubusercontent.com/ghcr1/blobs/sha256:4f387cc0993f868f31cd76483051a58420f80f57cf4626afc4b881d2a98959bb?se=2022-05-25T14%3A45%3A00Z&sig=W0EWqhIPIbU2p8RqfuSHqd0WtDjGYD14Atbj10gaPvE%3D&sp=r&spr=https&sr=b&sv=2019-12-12
2022-05-25T14:38:30.3848500Z ==> Pouring bash--5.1.16.big_sur.bottle.tar.gz
2022-05-25T14:39:09.7708830Z 🍺  /usr/local/Cellar/bash/5.1.16: 157 files, 10.9MB
2022-05-25T14:39:09.8268290Z ==> Running `brew cleanup bash`...
2022-05-25T14:39:09.8269400Z Disable this behaviour by setting HOMEBREW_NO_INSTALL_CLEANUP.
2022-05-25T14:39:09.8270090Z Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
2022-05-25T14:39:21.5336590Z ##[group]Run echo "PATHX=$(xcode-select --print-path)/Toolchains/XcodeDefault.xctoolchain/usr/lib/" >> $GITHUB_ENV
2022-05-25T14:39:21.5337310Z echo "PATHX=$(xcode-select --print-path)/Toolchains/XcodeDefault.xctoolchain/usr/lib/" >> $GITHUB_ENV
2022-05-25T14:39:21.5391180Z ##[error]Second path fragment must not be a drive or UNC name. (Parameter 'expression')
2022-05-25T14:39:21.5610240Z Cleaning up orphan processes

Runner and Worker's Diagnostic Logs

Start with .github/workflows/test.yml from the linked reproduction and run the workflow on macOS 11.6.5 using runner version 2.291.1. Follow the log entry after writing to $GITHUB_ENV and inspect how the runner produces the path-fragment error. Done means the error is understandable to macOS users and reports the problematic value.