ci: could use `iffy/install-nim` for every Nim version

[ee7]

Historically, the track has used alaviss/setup-nim to run tests with nightly Nim releases, and iffy/install-nim for stable Nim releases:

nim/.github/workflows/exercises.yml

Lines 32 to 45 in 8fa077f

	- name: Install Nim (devel)
	if: matrix.nim == 'devel'
	uses: alaviss/setup-nim@f81f2a6d1505ab32f440ec9d8adbb81e949d3bf0 # 0.1.1
	with:
	path: 'nim'
	version: ${{ matrix.nim }}
	- name: Install Nim (non-devel)
	if: matrix.nim != 'devel'
	uses: iffy/install-nim@dcaa84c8d3feb17cd7cc10bd124d1658888d9c0e
	with:
	version: "binary:${{ matrix.nim }}"
	env:
	GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

But I believe that iffy/install-nim has the ability to use the Nim nightlies too, with something like:

nightly:https://github.com/nim-lang/nightlies/releases/tag/latest-version-2-0