`cargo` (via `build_crate`) cannot create temp dir — sporadic

[szilardszaloki]

FAILED: wasm_pack/release/wasm-pack.exe 
C:/ws/x64-nightly/src/brave/vendor/depot_tools/bootstrap-2@3_11_8_chromium_35_bin/python3/bin/python3.exe ../../build/gn_run_binary.py C:/ws/x64-nightly/src/third_party/rust-toolchain/bin/cargo.exe build --quiet --release --manifest-path C:/ws/x64-nightly/src/brave/tools/crates/Cargo.toml --config C:/ws/x64-nightly/src/brave/tools/crates/.cargo/config.toml --target-dir C:/ws/x64-nightly/src/out/Release/wasm_pack --frozen --package wasm-pack 
error: couldn't create a temp dir: Access is denied. (os error 5) at path "C:\\Users\\ADMINI~1\\AppData\\Local\\Temp\\1\\rustcyBVkMC"
error: could not compile `smallvec` (lib) due to 1 previous error
C:/ws/x64-nightly/src/third_party/rust-toolchain/bin/cargo.exe failed with exit code 101

Likely related to rust-lang/rust#138475.
Set TMPDIR/TEMP/TMP to root_gen_dir for build_crate.