Prisma should check for generator binaries in $PATH

[steebchen]

When writing custom generators, you can specify an absolute or relative binary path for the "provider" value. However, the path resolution does not respect binaries in $PATH, which would make things easier for custom generators, because they can depend on globally installed binaries.

The following generator configuration does not work:

generator photon {
	provider = "photongo"
	output = "./photon/photon_gen.go"
	package = "photon"
}