legacy/nix-build: create various temporary directories into a known tempdir

Fixes fj#940.

When running `nix-shell`, the `$NIX_BUILD_TOP` environment variable is
set to `$TMPDIR` or `/tmp`.

nixpkgs stdenv uses $NIX_BUILD_TOP to create `$NIX_BUILD_TOP/env-vars`
which contains all the environment variables set by stdenv. This is used
for debugging purposes in combination with `--keep-failed` to reload the
bash environment of a derivation.

`$TMPDIR` is often unset, therefore, `/tmp/env-vars` was constantly
being created. On a multi-user system or, when you run Lix as root, you
might create a `/tmp/env-vars` with different permission bits.

As a result, `nix-shell` can cease to function because that file will
fail creation for an unprivileged user for example.

fj#940 rightfully remark that the code is not consistent between
nix3-develop and nix-shell and it should be reworked.

Change-Id: Iddf15945385d8bd497b2800b37fee5e1f97689b7
Signed-off-by: Raito Bezarius <raito@lix.systems>

doc/manual/rl-next/shell-build-dir-in-tempdir.md

@@ -0,0 +1,13 @@
+---
+synopsis: "`nix-shell` default shell directory is not `/tmp` anymore for `$NIX_BUILD_TOP`"
+cls: []
+issues: [fj#940]
+category: "Fixes"
+credits: [raito]
+---
+
+Previously, Lix `nix-shell`s could exit non-zero status when `stdenv`'s `dumpVars` phase failed to write to `$NIX_BUILD_TOP/env-vars`, despite `dumpVars` being intended as a debugging aid.
+
+This happens when `TMPDIR` is not set and defaults therefore to `/tmp`, resulting in a `/tmp/env-vars` global file that every `nix-shell` wants to write.
+
+We fix this issue by reusing a pre-created, unique, and writable location, as the build top directory, avoiding shell exiting from write failures silently.

lix/legacy/nix-build.cc

@@ -188,6 +188,7 @@ static void main_nix_build(AsyncIoRoot & aio, std::string programName, Strings a
         throw UsageError("'-p' and '-E' are mutually exclusive");
 
     AutoDelete tmpDir(createTempDir("", myName));
+    AutoDelete buildTopTmpDir(createTempDir(tmpDir, "build-top"));
     if (outLink.empty())
         outLink = (Path) tmpDir + "/result";
 
@@ -419,7 +420,8 @@ static void main_nix_build(AsyncIoRoot & aio, std::string programName, Strings a
         }
 
         // Don't use defaultTempDir() here! We want to preserve the user's TMPDIR for the shell
-        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = getEnvNonEmpty("TMPDIR").value_or("/tmp");
+        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] =
+            getEnvNonEmpty("TMPDIR").value_or(buildTopTmpDir);
         env["NIX_STORE"] = store->config().storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);