On CI, we have been running into OOM issues when running the test
suite on Windows for quite some time.
Unfortunately, we are very close to having the same issues on Linux
as well. Some additional comptime work immediately makes these builds
fail as well.
Add a new `test-toolchain` step, that tests everything except `std.*`
and documentation.
On CI, call `test-toolchain`, `test-std` and `docs` separately
instead of the `test` big hammer that emcompasses all of them.
Change the special case we made for Windows to the same code as other
platforms.
This is a stopgap measure that stage2 will eventually make useless.
Until then, it gives us some headroom.
Change `linux_script` by the way to only output the log of failing
steps. This shrinks the Linux CI log from a bazilion lines down to
something more humanely manageable.
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