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
  Build
failed #267496
build

00:07:15

 
Name Stage Failure
failed
build Build
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ERROR: Job failed: exit code 1