feat: add emulator constraint for QEMU
Previously, we were registering the QEMU toolchain against any AMD64 Linux, which would select that toolchain if running one of the workflows
(e.g. labgrid_genrule
) without specifying a platform. We now require consumers to opt-in to using QEMU by using the constraint @rules_labgrid//constraint/emulator:qemu
.
The new constraint lives under a new top-level directory, as it's not specific to rules_labgrid
and could be extracted into a separate
module in the future.