Improve build endpoint in Bulk Expression Atlas

[ke4]

Changes:

Use gradle-git-properties plugin to provide build information that could be useful to a developer for debugging purposes. Currently it returns this information:

{
   "Build version": "1.0.0",
   "Git branch": "develop",
   "Latest commit ID": "809f75d6ad696efc2e7fb7edc0b20b95620de1a4",
   "Latest commit message": "Update docker-compose.yml for postgres to follow changes in db-scxa repo"
}

We can add more information later if needed.

This PR implementing this issue: https://github.com/ebi-gene-expression-group/atlas-web-single-cell/issues/365

Additional changes:

• I had to fix the declaration of the experiment files location in the single test executor.
• Also removed an unused variable (SCHEMA_VERSION) in the single test executor.

[ke4]

I am going to merge this as I was doing the same modification like in SCXA and that was approved by @upendrakumbham .