vmtest: use same Go version to compile initramfs and Go tests
vmtest: use same Go version to compile initramfs and Go tests

This makes sure the u-root initramfs as well as the Go tests to run in
the VM are compiled with the same Go version as the test runner.

Previously, if one did (cd integration/gotests && go1.13 test), the test
runner and initramfs would be compiled with go1.13, but the Go VM tests
would be compiled with `go` found in $PATH.

Signed-off-by: Chris Koch <cd2178739c1fb8f241acc6c7bd5abe02c150d7e5@google.com>

u-root/u-root,u-root/u-root,u-root/u-root