rustc needs python2

[ken@…]

It seems I must have got lucky when using python3 for rustc-1.29. The current version (1.31.1) craps out very early with a segmentation fault if python3 is specified:

Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/scratch/ken/rustc-1.31.1-src/build/bootstrap/debug/rustc -
--crate-name ___ --print=file-names -C link-args=-lffi --target x86_64-unknown-linux-gnu
--crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib
--crate-type proc-macro` (signal: 11, SIGSEGV: invalid memory reference)
command did not execute successfully:
"/scratch/ken/rustc-1.31.1-src/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target"
"x86_64-unknown-linux-gnu" "-j" "4" "--release" "--features" "panic-unwind jemalloc backtrace"
"--manifest-path" "/scratch/ken/rustc-1.31.1-src/src/libstd/Cargo.toml" "--message-format" "json"
expected success, got: exit code: 101
thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1115:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failed to run: /scratch/ken/rustc-1.31.1-src/build/bootstrap/debug/bootstrap build
Build completed unsuccessfully in 0:00:02

[ken@…]

Fixed, r20927.