fix outdir for windows (for good this time...)

[timotheecour]

sorry for the noise @jangko, looks like the previous attempts didn't work; I've now bitten the bullet and found a windows VM to test this on and make sure it all works; and for extra check I've added this to https://github.com/nim-lang/Nim/pull/13382 to make sure PR is green with this change:

pkg "msgpack4nim", "sh -c 'git checkout pr_fix_outdir3 && nimble test'", url = "https://github.com/timotheecour/msgpack4nim"

so let's wait for nim CI https://github.com/nim-lang/Nim/pull/13382 to be green before merging this, then I'll undo the above line from the nim PR.

note

• --outdir:'$projectdir' unfortunately doesn't seem to work on windows (this shouldn't be hard to fix in Nim); so I'm using --outdir:tests here

[jangko]

closing and reopening to trigger the CI.

[timotheecour]

but before merging, let's please wait for https://github.com/nim-lang/Nim/pull/13382 to be green :)

[jangko]

sure

[timotheecour]

https://github.com/nim-lang/Nim/pull/13382 is green at last! thanks for your patience