On CI we test on go1.23.x and go1.24.x, so if we always upgrade
to the latest go1.24.x, that will cause garble to complain
when running on go1.23.x:
garble was built with "go1.23.7" and can't be used with the newer "go1.24.1"
Moreover, the test hard-coded go1.24.1, which is currently the latest
go1.24.x but will not be for long, so this test was brittle.
We call `go list` to collect information about all the packages
to obfuscate and build, which is crucial to be able to perform
obfuscation of names used across packages.
However, when GOTOOLCHAIN causes a toolchain upgrade,
we must ensure that we use the upgraded Go tool;
otherwise we are mixing information from different toolchain versions.
Fixes#934.
When we build the patched cmd/link binary for use by garble,
we perform this build in a temporary directory so that the Go module
from the user does not get in the way.
When the user module made us upgrade the toolchain per GOTOOLCHAIN,
leaving that module's directory stops upgrading the toolchain,
so we patch a newer toolchain and build it with an older toolchain.
This is largely harmless, but it makes the newer toolchain think
it is actually an older toolchain, which leads to those pesky
"linker object header mismatch" version errors.
Updates #934.
Daniel Martí