add warning if requesting x64 on apple silicon runners

[IanButterworth]

I think this might be happening quite a bit in the wild.

i.e. JuliaLang/julia#55878

[IanButterworth]

I say in the wild... but it's the case on this CI... 😂

[vtjnash]

This seems misleading, since leaving it unspecified causes it to throw an exception and fail:

setup-julia/src/setup-julia.ts

Lines 61 to 63 in 5c9647d

	if (!originalArchInput) { // if `originalArchInput` is an empty string
	throw new Error(`Arch input must not be null`)
	}

https://github.com/vtjnash/Glob.jl/actions/runs/11898442357/job/33154951261?pr=44

[IanButterworth]

Unspecified as in not an empty string, just not set at all.

[DilumAluthge]

If you do this:

- uses: julia-actions/setup-julia@desiredtag
  with:
    version: '1.10'

Then setup-julia will default to the architecture of the runner.

---

In contrast, if you do this:

- uses: julia-actions/setup-julia@desiredtag
  with:
    version: '1.10'
    arch: ''

Then setup-julia will (correctly) throw an error.

[vtjnash]

If you use a matrix and don't specify arch, then it will get an empty string here

[DilumAluthge]

The following two are equivalent:

- uses: julia-actions/setup-julia@desiredtag
  with:
    version: '1.10'

- uses: julia-actions/setup-julia@desiredtag
  with:
    version: '1.10'
    arch: 'default'

[vtjnash]

The documentation says it defaults to '', though I agree that using "default" instead works

setup-julia/README.md

Lines 56 to 57 in 5c9647d

	# Defaults to the architecture of the runner executing the job.
	arch: ''

[DilumAluthge]

If you use a matrix, you shouldn't be passing an empty string ''. You should pass 'default' instead.

[IanButterworth]

Yeah, the readme is misleading. They're all like that, note the default behaviors are written above the empty string examples. Not sure what the clearest way to write them all is.

[DilumAluthge]

We should fix up the README. For inputs that are optional (like arch), we should put a reasonable default in there.

[DilumAluthge]

#302