addNuGetDeps: move TMPDIR handling out of fetch-deps

We now handle creating TMPDIR and setting the working directory outide
of nix-shell.  This allows nix-shell to set NIX_BUILD_TOP correctly.

pkgs/build-support/dotnet/add-nuget-deps/default.nix

@@ -79,7 +79,12 @@ attrs
 
         in
         writeShellScript "${finalPackage.name}-fetch-deps" ''
-          NIX_BUILD_SHELL="${runtimeShell}" exec ${nix}/bin/nix-shell \
+          set -eu
+          export TMPDIR
+          TMPDIR=$(mktemp -d -t fetch-deps-${finalPackage.name}.XXXXXX)
+          trap 'chmod -R +w "$TMPDIR" && rm -fr "$TMPDIR"' EXIT
+          cd "$TMPDIR"
+          NIX_BUILD_SHELL="${runtimeShell}" {nix}/bin/nix-shell \
             --pure --run 'source "${innerScript}"' "${drv}"
         '';
     };

pkgs/build-support/dotnet/add-nuget-deps/fetch-deps.sh

@@ -1,20 +1,18 @@
 set -e
 
-tmp=$(mktemp -d)
-trap 'chmod -R +w "$tmp" && rm -fr "$tmp"' EXIT
-
-HOME=$tmp/.home
-export TMPDIR="$tmp/.tmp"
-mkdir "$HOME" "$TMPDIR"
-cd "$tmp"
+HOME=$NIX_BUILD_TOP/home
+mkdir "$HOME"
 
 export NIX_SSL_CERT_FILE=@cacert@/etc/ssl/certs/ca-bundle.crt
 
 genericBuild
 
 depsFile=$(realpath "${1:-@defaultDepsFile@}")
-tmpFile="$tmp"/deps.nix
-echo -e "# This file was automatically generated by passthru.fetch-deps.\n# Please dont edit it manually, your changes might get overwritten!\n" > "$tmpFile"
-@nugetToNix@/bin/nuget-to-nix "${NUGET_PACKAGES%/}" >> "$tmpFile"
-mv "$tmpFile" "$depsFile"
+
+(
+    echo -e "# This file was automatically generated by passthru.fetch-deps.\n# Please dont edit it manually, your changes might get overwritten!\n"
+    @nugetToNix@/bin/nuget-to-nix "${NUGET_PACKAGES%/}"
+) > deps.nix
+
+mv deps.nix "$depsFile"
 echo "Succesfully wrote lockfile to $depsFile"