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 <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
diff --git a/tests/kokoro/gcp_ubuntu/continuous.sh b/tests/kokoro/gcp_ubuntu/continuous.sh
index 96e980a..0f5b872 100755
--- a/tests/kokoro/gcp_ubuntu/continuous.sh
+++ b/tests/kokoro/gcp_ubuntu/continuous.sh
@@ -6,6 +6,7 @@
 set -x # Display commands being run.
 
 # Specify we want to build with GCC 7
+sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo apt-get update
 sudo apt-get install -y gcc-7 g++-7
 sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 100 --slave /usr/bin/g++ g++ /usr/bin/g++-7