TSCBasic: avoid using C library functions for environment

Prefer to use the Win32 APIs for environment management.  Note that this
is going to break any user of libc on Windows.  Setting environment
variables through this function will not be reflected in the C library
only the Win32 APIs.  Furthermore, we use the unicode variants always as
the unicode and ANSI environment may diverge as not all unicode (UTF-16)
is translatable to ANSI, which is documented at [1].

[1] https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/getenv-wgetenv?view=msvc-170&viewFallbackFrom=vs-2019.

Author: compnerd

Sources/TSCBasic/Process/ProcessEnv.swift

@@ -26,8 +26,12 @@ public enum ProcessEnv {
     /// Set the given key and value in the process's environment.
     public static func setVar(_ key: String, value: String) throws {
       #if os(Windows)
-        guard TSCLibc._putenv("\(key)=\(value)") == 0 else {
-            throw SystemError.setenv(Int32(GetLastError()), key)
+        try key.withCString(encodedAs: UTF16.self) { pwszKey in
+          try value.withCString(encodedAs: UTF16.self) { pwszValue in
+            guard SetEnvironmentVariableW(pwszKey, pwszValue) else {
+              throw SystemError.setenv(Int32(GetLastError()), key)
+            }
+          }
         }
       #else
         guard TSCLibc.setenv(key, value, 1) == 0 else {
@@ -40,7 +44,9 @@ public enum ProcessEnv {
     /// Unset the give key in the process's environment.
     public static func unsetVar(_ key: String) throws {
       #if os(Windows)
-        guard TSCLibc._putenv("\(key)=") == 0 else {
+        guard key.withCString(encodedAs: UTF16.self, {
+          SetEnvironmentVariableW($0, nil)
+        }) else {
             throw SystemError.unsetenv(Int32(GetLastError()), key)
         }
       #else
@@ -70,9 +76,7 @@ public enum ProcessEnv {
     public static func chdir(_ path: AbsolutePath) throws {
         let path = path.pathString
       #if os(Windows)
-        guard path.withCString(encodedAs: UTF16.self, {
-            SetCurrentDirectoryW($0)
-        }) else {
+        guard path.withCString(encodedAs: UTF16.self, SetCurrentDirectoryW) else {
             throw SystemError.chdir(Int32(GetLastError()), path)
         }
       #else