libplacebo-ubuntu-cosmic: make more useful for debugging

Install gdb and coredumpctl, hopefully the combination will let me
figure out why the libplacebo test suite segfaults here. Also build
shaderc with debug symbols.
1 job for master in 7 minutes and 15 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #267496


Name Stage Failure
build Build
61a0b6b47641: Retrying in 2 seconds
fe3daf0c57b2: Retrying in 2 seconds
9f2ab8400711: Retrying in 1 second
54efbff3ecfa: Retrying in 1 second
0cb99ab21550: Retrying in 1 second
61a0b6b47641: Retrying in 1 second
fe3daf0c57b2: Retrying in 1 second
blob upload unknown
ERROR: Job failed: exit code 1