community/crystal: Use CRYSTAL_CONFIG_BUILD_COMMIT to specify build commit

Running crystal --version (on edge, 0.30.1-r1) gives the following output:

$ crystal --version
Crystal 0.30.1 [284dccd1b9] (2019-08-22)

The build commit (after the version number) incorrectly references the commit of the abuild package (http://git.alpinelinux.org/cgit/aports/commit/?id=284dccd1b95258f680a1aa3f9744f69646e033d1), because it defaults to git rev-parse --short HEAD. But it should reference the Crystal commit. Output from the official Crystal build:

$ crystal --version
Crystal 0.30.1 [5e6a1b672] (2019-08-12)

The fix defines CRYSTAL_CONFIG_BUILD_COMMIT with the commit hash from the Crystal repo.