test: don't use `-cpu max` in GH Actions

test: don't use `-cpu max` in GH Actions

There appears to be an issue with newer QEMU versions (spotted with Arch
Linux and C9S containers) which causes the respective GH Action to hang
when booting a QEMU VM in combination with the `-cpu max` parameter.

During (a particularly painful) debugging session I once managed to get
some output from such "frozen" machine (using `earlycon` and
`earlyprintk` kernel cmdline options), and in that particular case the
VM died with a trap caused by an invalid opcode.

I couldn't reproduce this locally, only in GH Actions environment with
Arch Linux and C9S containers. Also, so far I haven't found out which
specific CPUID flag causes this, but using the `IvyBridge-v2` feature
set seems to mitigate the issue.