Migration to new way to set env var and path in GH Actions

[tesuji]

Deprecation announcement blog: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

New way: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files

An affected file:

rustup/ci/actions-templates/linux-builds-template.yaml

Lines 82 to 83 in 243b453

	echo "::add-path::$HOME/.cargo/bin"
	echo "::set-env name=TARGET::${{matrix.target}}"

[tesuji]

Maybe fixed by #2513.

[kinnison]

This has been merged now.