cmd/racebuild: use yum for linux/s390x support
cmd/racebuild: use yum for linux/s390x support

linux-s390x-ibm uses a RHEL derivative and not a Debian one [1].
Therefore use yum instead of apt-get.

[1] https://go-review.googlesource.com/c/go/+/336549/comments/eb61ef44_7b74dba0

Change-Id: I0a051745e1417ea6c9282a63bc0975705e63ad34
Reviewed-on: https://go-review.googlesource.com/c/build/+/399996
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Auto-Submit: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>

golang/build,golang/build,golang/build,golang/build