This commit skips the arch checks if the experimental "bootstrap"
option is used. The main use-case of this option is to bootstrap
a foreign architecture so just assume that and skip arch checks
when set.
This allows to write:
```
$ IMAGE_BUILDER_EXPERIMENTAL=bootstrap=ghcr.io/mvo5/fedora-buildroot:41 \
./image-builder build --arch=riscv64 minimal-raw --distro=fedora-41
```
and do a riscv64 cross arch build.
8635a22ad9