Summary:
The `go build` tool can accept linker options that dynamically set
variable values at build time. Let's make use of that in our `Makefile`
and `debian/rules` to know and expose meta data such as version and Git
commit at runtime.

Test Plan:
Run `make bin/blubber && bin/blubber --version` and verify that it outputs
"0.0.1-[git head commit]". Build and install the debian package and verify
the same using the installed binary.

Reviewers: thcipriani, hashar, Joe, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D816