Kokoro: Add the ppa:ubuntu-toolchain-r/test apt-repository
Kokoro: Add the ppa:ubuntu-toolchain-r/test apt-repository

Bug: None - Fix Kokoro presubmits.
Change-Id: I16074e733edd257647840d1bf51ebdc14dd4fb89
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43431
Reviewed-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader