Makefile: Add section for environment variables

GIT_VERSION can be set to, for instance, run tests with a specific git version. Add a section to make help to display this.

make help output:

usage: make [<target>...] [<variable>=<value>...]

These are the available targets:

  all                    Default target which builds Gitaly.
  bench                  Run Go benchmarks.
  build-bundled-git      Build bundled Git binaries.
  build                  Build Go binaries and install required Ruby Gems.
  clean                  Clean up build artifacts.
  cover                  Generate coverage report via Go tests.
  debug-test-go          Run Go tests in delve debugger.
  dump-database-schema   Dump the clean database schema of Praefect into a file.
  format                 Run Go formatter and adjust imports.
  git                    Build Git.
  help                   Print help about available targets and variables.
  install-bundled-git    Install bundled Git binaries. The target directory can be modified by setting PREFIX and DESTDIR.
  install                Install Gitaly binaries. The target directory can be modified by setting PREFIX and DESTDIR.
  libgit2                Build libgit2.
  lint                   Run Go linter.
  notice                 Regenerate the NOTICE file.
  proto                  Regenerate protobuf definitions.
  race-go                Run Go tests with race detection enabled.
  rspec                  Run Ruby tests.
  rubocop                Run Rubocop.
  test-go                Run Go tests.
  test-with-praefect     Run Go tests with Praefect.
  test                   Run Go and Ruby tests.
  verify                 Verify that various files conform to our expectations.

These are common variables which can be overridden in config.mak:

  DEBUG_OPTIONS           Test options passed to `dlv test`.
  FIPS_MODE               FIPS_MODE controls whether to build Gitaly and dependencies in FIPS mode. Set this to a non-empty value to enable it.
  GIT_BUILD_OPTIONS       Build options for Git.
  GIT_PREFIX              The prefix where Git will be installed to.
  LIBGIT2_BUILD_OPTIONS   Build options for libgit2.
  PREFIX                  The prefix where Gitaly binaries will be installed to. Binaries will end up in ${PREFIX}/bin by default.
  PROTOC_BUILD_OPTIONS    Build options for protoc.
  TEST_COVERAGE_DIR       Specify the output directory for test coverage reports.
  TEST_FORMAT             Specify the output format used to print tests ["standard-verbose", "standard-quiet", "short"]
  TEST_OPTIONS            Test options passed to `go test`.
  TEST_PACKAGES           List of Go packages which shall be tested. Go packages to test when using the test-go target.
  TEST_TMP_DIR            Directory where all runtime test data is being created.

These are environment variables which can be used:

GIT_VERSION  Sets the git version.