Fix logic of SWIFT_DRIVER_TOOLNAME_EXEC override

The previous searching logic doesn't match the description, and will
instead fail if the specified path doesn't exist. This commit changes
the behavior to fallthrough if the path doesn't exist, similar to
legacy driver and SwiftPM.

Author: stevapple

Sources/SwiftDriver/Toolchains/Toolchain.swift

@@ -206,8 +206,9 @@ extension Toolchain {
   /// looks in the `executableDir`, `xcrunFind` or in the `searchPaths`.
   /// - Parameter executable: executable to look for [i.e. `swift`].
   func lookup(executable: String) throws -> AbsolutePath {
-    if let overrideString = envVar(forExecutable: executableName(executable)) {
-      return try AbsolutePath(validating: overrideString)
+    if let overrideString = envVar(forExecutable: executableName(executable)),
+       let path = try? AbsolutePath(validating: overrideString) {
+      return path
     } else if let toolDir = toolDirectory,
               let path = lookupExecutablePath(filename: executableName(executable), currentWorkingDirectory: nil, searchPaths: [toolDir]) {
       // Looking for tools from the tools directory.