Fix testExplicitModuleBuildEndToEnd by passing in the SDK path explicitly.

As-is, the test is sensitive to the environment and is currently failing with:
```
ld: library not found for -lobjc
```
Passing in the sdk path explicitly should make it pass and be more resilient.

Author: artemcm

Package.resolved

@@ -557,12 +557,18 @@ final class ExplicitModuleBuildTests: XCTestCase {
         $0 <<< "import G;"
       }
 
+      let executor = try SwiftDriverExecutor(diagnosticsEngine: DiagnosticsEngine(handlers: [Driver.stderrDiagnosticsHandler]),
+                                             processSet: ProcessSet(),
+                                             fileSystem: localFileSystem,
+                                             env: ProcessEnv.vars)
+      let sdkPath = try executor.checkNonZeroExit(
+          args: "xcrun", "-sdk", "macosx", "--show-sdk-path").spm_chomp()
       let packageRootPath = URL(fileURLWithPath: #file).pathComponents
           .prefix(while: { $0 != "Tests" }).joined(separator: "/").dropFirst()
       let testInputsPath = packageRootPath + "/TestInputs"
       let cHeadersPath : String = testInputsPath + "/ExplicitModuleBuilds/CHeaders"
       let swiftModuleInterfacesPath : String = testInputsPath + "/ExplicitModuleBuilds/Swift"
-      var driver = try Driver(args: ["swiftc",
+      var driver = try Driver(args: ["swiftc", "-sdk", sdkPath,
                                      "-I", cHeadersPath,
                                      "-I", swiftModuleInterfacesPath,
                                      "-experimental-explicit-module-build",