Fix issue with running test on windows w/no-parallel

The AsyncProcess Tests were using a TSCTestSupport method to temporarly change the PATH env but seems to have a bug where it removes the variable instead of restoring resulting in any future test needing PATH to fail.  This switch it to using the same method that is in SwiftPM Environment.swft

Author: daveinglis

Tests/BasicsTests/AsyncProcessTests.swift

@@ -20,7 +20,6 @@ import struct TSCBasic.ByteString
 import struct TSCBasic.Format
 import class TSCBasic.Thread
 import func TSCBasic.withTemporaryFile
-import func TSCTestSupport.withCustomEnv
 
 #if os(Windows)
 let catExecutable = "type"
@@ -168,7 +167,7 @@ final class AsyncProcessTests: XCTestCase {
             #endif
             try localFileSystem.writeFileContents(tempExecutable, bytes: exitScriptContent)
 
-            try withCustomEnv(["PATH": tmpdir.pathString]) {
+            try Environment.makeCustom(["PATH": tmpdir.pathString]) {
                 XCTAssertEqual(AsyncProcess.findExecutable("nonExecutableProgram"), nil)
             }
         }
@@ -184,7 +183,7 @@ final class AsyncProcessTests: XCTestCase {
 
             """)
 
-            try withCustomEnv(["PATH": tmpdir.pathString]) {
+            try Environment.makeCustom(["PATH": tmpdir.pathString]) {
                 do {
                     let process = AsyncProcess(args: "nonExecutableProgram")
                     try process.launch()

Tests/BasicsTests/Environment/EnvironmentTests.swift

@@ -144,6 +144,19 @@ struct EnvironmentTests {
         #expect(Environment.current[key] == nil)
     }
 
+    @Test
+    func makeCustomPathEnv() async throws {
+        let value = "TestValue"
+
+        var customEnvironment: Environment = .current
+        let origPath = customEnvironment[.path]
+
+        try Environment.makeCustom(["PATH": "/foo/bar"]) {
+            #expect(Environment.current[.path] == "/foo/bar")
+        }
+        #expect(Environment.current[.path] == origPath)
+    }
+
     /// Important: This test is inherently race-prone, if it is proven to be
     /// flaky, it should run in a singled threaded environment/removed entirely.
     @Test