`mlserver build` fails if requirements.txt includes a git path

[isaac-smothers]

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As git isn't installed on the mlserver images, any references to git paths in your requirements.txt will fail mlserver build commands.

Reproduce:

• Specify a git path in your requirements.txt like git+https://github.com/x/x.git
• Run mlserver build
• ERROR: Error [Errno 2] No such file or directory: 'git' while executing command git version