Kokoro: Fix ubuntu bots.

Install gcc with aptitude instead of apt-get.

Mirroring workaround from https://github.com/google/amber/pull/844

Bug: none. Fixing kokoro.
Change-Id: I4b57653a539ef78bf037d508dedc47f339c5257d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44130
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@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 9b6f2ce..f09a0cf 100755
--- a/tests/kokoro/gcp_ubuntu/continuous.sh
+++ b/tests/kokoro/gcp_ubuntu/continuous.sh
@@ -8,7 +8,7 @@
 # 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 aptitude 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
 sudo update-alternatives --set gcc "/usr/bin/gcc-7"