Allow overriding GITALY_SERVER_VERSION in the env
What does this MR do and why?
The version of Gitaly is specified in the GITALY_SERVER_VERSION file in the root of this project. For testing purposes it might be useful if this can be easily overridden.
This change will read $GITALY_SERVER_VERSION from the environment, and if it's set it ignores the file but uses the version specified in the environment variable.
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