ci: PACKAGES hack doesn't work on Windows

[kolyshkin]

Apparently the hack introduced by #145 (using custom PACKAGES list for older Go versions) is not working and never worked on Windows.

Here is an example of a run which has the full list of PACKAGES on Windows:

https://github.com/moby/sys/actions/runs/10298746983/job/28504536200

You can see it works fine on other platforms (Linux and Mac OS X).

[kolyshkin]

@thaJeztah I'm thinking about a way to add a kludge to skip running a command if go version in package's go.mod is newer than that of go version.

[kolyshkin]

The alternative is to drop go 1.18 support 👯

[thaJeztah]

Hm.... interesting;

So the PACKAGES env-var is ignored in the Makefile on Windows; is that a difference between Make used on Windows and Linux?

Also curious why CI passed even though there's clearly some error; is that because of our use of a sub shell ((cd $p; go mod tidy);)?