[TSCBasic] Allow overriding the temporary directory

SwiftPM used to allow overriding the temporary directory using the
TMPDIR variable. This facility was removed recently in favor of
NSTemporaryDirectory() but that method doesn't respect TMPDIR on macOS.

<rdar://problem/55147926>

Author: aciidgh

Sources/TSCBasic/TemporaryFile.swift

@@ -45,14 +45,23 @@ private extension TempFileError {
 ///
 /// - Returns: Path to directory in which temporary file should be created.
 public func determineTempDirectory(_ dir: AbsolutePath? = nil) throws -> AbsolutePath {
-    // FIXME: Add other platform specific locations.
-    let tmpDir = dir ?? AbsolutePath(NSTemporaryDirectory())
+    let tmpDir = dir ?? cachedTempDirectory
     guard localFileSystem.isDirectory(tmpDir) else {
         throw TempFileError.couldNotFindTmpDir
     }
     return tmpDir
 }
 
+/// Returns temporary directory location by searching relevant env variables.
+///
+/// Evaluates once per execution.
+private var cachedTempDirectory: AbsolutePath = {
+    let override = ProcessEnv.vars["TMPDIR"] ?? ProcessEnv.vars["TEMP"] ?? ProcessEnv.vars["TMP"]
+    if let path = override.flatMap({ try? AbsolutePath(validating: $0) }) {
+        return path
+    }
+    return AbsolutePath(NSTemporaryDirectory())
+}()
 
 /// The closure argument of the `body` closue of `withTemporaryFile`.
 public struct TemporaryFile {