debian: set GOCACHE dir during build to fix FTBFS on eoan
The eoan sbuild fails with the following error: ``` failed to initialize build cache at /sbuild-nonexistent/.cache/go-build: mkdir /sbuild-nonexistent: permission denied ```
This can be fixed by setting GOCACHE explicitly - if unset it is derrived from $HOME which is set to an impossible location in the sbuild environment.