SVNEXE Provider: enable setting env, path, or full path to svn.exe

We need to use the svn exe provider in a context where the environment path does not contain svn.exe.

We can programatically determine the location of svn.exe and need to pass this location into the provider so that either,

• we can alter the path env variable used by the provider when running a command line (preferred)
• or, tell the provider the full path to the svn.exe to use,