Use DC environment variable as default D compiler

dub now respects the `DC` environment variable, meaning that `DC=ldc2 dub build` will behave as `dub build --compiler=ldc2`.
In case both are supplied, the `--compiler` switch still has priority.
Note that when DUB recursively invokes itself, for example in `preGenerateCommands`,
it sets the `DC` variable to the compiler it is using, meaning that nested dub invocation will now use the same compiler.