cmd/buildlet: execute shell scripts through bash on mobile platforms
cmd/buildlet: execute shell scripts through bash on mobile platforms

iOS and Android can't execute shell directly. Use bash instead.

Updates golang/go#31722

Change-Id: I288f4af2abe2bed268b7c219608067047d2e0d9c
Reviewed-on: https://go-review.googlesource.com/c/build/+/174997
Run-TryBot: Elias Naur <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@eliasnaur.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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