Fix merge of FunctionCall vars argument in FetchBuild

[eramongodb]

A fix to a bug that has not yet been encountered in the C Driver.

If a custom vars={...} argument is given to FetchBuild.call(build_name="abc", vars={...}), e.g. to define additional environment variables, the config generator will fail due to the vars argument being specified multiple times:

TypeError: config_generator.etc.function.Function.default_call() got multiple values for keyword argument 'vars'

This PR uses .update() to ensure if vars is given, that the build_name will correctly be merged into vars instead of triggering a conflict.