Fixing maven repo path separator char

[marfx000]

The bug happens only if:

• "Patch mainTemplate.gradle" setting is enabled;
• You are resolving dependencies on Windows platform;

Examples of bad repo path string in mainTemplate.gradle:
file:///C:/Work/UnityProject\Assets/GeneratedLocalRepo/Firebase/m2repository

This happened because Path.Combine uses "" as path separator for
windows platforms. But gradle actually expects not a regular path
but file URI, where "/" must be used as path separator.

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[marfx000]

@googlebot I signed it!