Tests: mostly repair the SwiftDriverTests on Windows

With this set of changes, we are now down to 1 failure on the
SwiftDriverTests on Windows. While we are not near the original state of
tests on Windows (the full test suite used to pass), this is a step in
the right direction.

Author: compnerd

Sources/SwiftDriver/Toolchains/Toolchain.swift

@@ -222,7 +222,7 @@ extension Toolchain {
               let path = lookupExecutablePath(filename: executableName(executable), currentWorkingDirectory: nil, searchPaths: [toolDir]) {
       // Looking for tools from the tools directory.
       return path
-    } else if let path = lookupExecutablePath(filename: executableName(executable), currentWorkingDirectory: nil, searchPaths: [try executableDir]) {
+    } else if let path = lookupExecutablePath(filename: executableName(executable), currentWorkingDirectory: fileSystem.currentWorkingDirectory, searchPaths: [try executableDir]) {
       return path
     } else if let path = try? xcrunFind(executable: executableName(executable)) {
       return path