isabelle: don't use /tmp as a temp directory

Isabelle uses `/tmp` no matter what on Unix platforms, apparently
because using an arbitrary `$TMPDIR` can cause "path too long" errors on
macOS (https://isabelle.in.tum.de/repos/isabelle/rev/ff92d6edff2c).

However, when not using the sandbox (i.e. on Darwin), this causes the
build to fail if its temp directory (`/tmp/isabelle-`) has already been
created by a previous build using a different build user. For example,
this is the cause of https://hydra.nixos.org/build/321069301.

So, this PR changes it to use `$TMPDIR` during the build of Isabelle
itself (but still use `/tmp` when using Isabelle normally). I had
initially hoped this would also enable Isabelle to build within the
Darwin sandbox, but something else seems to be wrong (I get an opaque
`I/O error: Operation not permitted`).

pkgs/applications/science/logic/tlaplus/tlaps.nix

@@ -31,9 +31,14 @@ stdenv.mkDerivation rec {
 
   installPhase = ''
     mkdir -pv "$out"
-    export HOME="$out"
+    export HOME="$TMPDIR"
     export PATH=$out/bin:$PATH
 
+    # Stop Isabelle trying to use `/tmp`.
+    user_home="$(isabelle getenv -b ISABELLE_HOME_USER)"
+    mkdir -p "$user_home/etc"
+    echo 'ISABELLE_TMP_PREFIX="$TMPDIR/isabelle"' > "$user_home/etc/settings"
+
     pushd zenon
     ./configure --prefix $out
     make

pkgs/by-name/is/isabelle/package.nix

@@ -266,6 +266,11 @@ stdenv.mkDerivation (finalAttrs: {
     export HOME=$TMP # The build fails if home is not set
     setup_name=$(basename contrib/isabelle_setup*)
 
+    # Stop Isabelle trying to use `/tmp`.
+    user_home="$(bin/isabelle getenv -b ISABELLE_HOME_USER)"
+    mkdir -p "$user_home/etc"
+    echo 'ISABELLE_TMP_PREFIX="$TMPDIR/isabelle"' > "$user_home/etc/settings"
+
     #The following is adapted from https://isabelle.sketis.net/repos/isabelle/file/Isabelle2021-1/Admin/lib/Tools/build_setup
     TARGET_DIR="contrib/$setup_name/lib"
     rm -rf "$TARGET_DIR"