Use executing binary path for install command

[KodrAus]

Closes #432

When installed as a dotnet tool, we distribute a platform-independent nupkg and the local dotnet toolchain assembles a shim binary over it. The DLL path in this case doesn't point anywhere near where the actual binary is.

I've tweaked it to use Environment.ProcessPath, which in my local testing here in Windows does point in the right direction. It'll need a check with the actual nupkg too.

[nblumhardt]

mcp install assumes that seqcli is a binary on $PATH; it might not be a bad idea to switch strategies to the one above there, too.