Use TMPDIR env var if available to get the temp dir name (#11443) [bugfix]

Additionally, use normalizePathEnd to suffix the dir name with "/" or
"\" as appropriate for the current OS.

Fixes https://github.com/nim-lang/Nim/issues/11439.

tests/stdlib/tos.nim

@@ -333,3 +333,15 @@ block ospaths:
   doAssert joinPath("", "lib") == "lib"
   doAssert joinPath("", "/lib") == unixToNativePath"/lib"
   doAssert joinPath("usr/", "/lib") == unixToNativePath"usr/lib"
+
+block getTempDir:
+  block TMPDIR:
+    # TMPDIR env var is not used if either of these are defined.
+    when not (defined(tempDir) or defined(windows) or defined(android)):
+      if existsEnv("TMPDIR"):
+        let origTmpDir = getEnv("TMPDIR")
+        putEnv("TMPDIR", "/mytmp")
+        doAssert getTempDir() == "/mytmp/"
+        putEnv("TMPDIR", origTmpDir)
+      else:
+        doAssert getTempDir() == "/tmp/"