The following environment variables are no longer supported:
```
ODASA_BUILD_ERROR_DIR
ODASA_CSHARP_LAYOUT
ODASA_SNAPSHOT
SEMMLE_DIST
SEMMLE_EXTRACTOR_OPTIONS
SEMMLE_PLATFORM_TOOLS
SEMMLE_PRESERVE_SYMLINKS
SOURCE_ARCHIVE
TRAP_FOLDER
```
Instead of only considering a fixed set of paths for `dotnet` and `mono`,
first attempt to lookup the paths based on the `PATH` environment variable.
This change also fixes a potential `System.IO.DirectoryNotFoundException` exception,
which could be thrown when the `shared/Microsoft.NETCore.App` folder was not
present.