mutation_test.sh fails due to a missing file or directory

[jdm]

RUSTFLAGS= bash ./etc/ci/mutation_test.sh
 in dir /home/servo/buildbot/slave/linux-rel-nogate/build (timeout 1200 secs)
 watching logfiles {}
 argv: ['RUSTFLAGS=', 'bash', './etc/ci/mutation_test.sh']
 environment:
  BUILD_MACHINE=<buildbot.process.properties.Property instance at 0x7f7207b10dd0>
  CARGO_HOME=/home/servo/.cargo
  CARGO_INCREMENTAL=0
  CCACHE=/usr/bin/ccache
  DISPLAY=:0
  HOME=/home/servo
  PATH=/home/servo/.cargo/bin:/home/servo/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
  PWD=/home/servo/buildbot/slave/linux-rel-nogate/build
  RUSTFLAGS=-Dwarnings
  RUST_BACKTRACE=1
  SCCACHE_IDLE_TIMEOUT=0
  SERVO_CACHE_DIR=/home/servo/.servo
  SHELL=/bin/bash
  TERM=linux
  UPSTART_INSTANCE=
  UPSTART_JOB=buildbot-slave
 using PTY: False
Upon execvpe RUSTFLAGS= ['RUSTFLAGS=', 'bash', './etc/ci/mutation_test.sh'] in environment id 140003549946216
:Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/process.py", line 430, in _fork
    environment)
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/process.py", line 508, in _execChild
    os.execvpe(executable, args, environment)
  File "/usr/lib/python2.7/os.py", line 353, in execvpe
    _execvpe(file, args, env)
  File "/usr/lib/python2.7/os.py", line 380, in _execvpe
    func(fullname, *argrest)
OSError: [Errno 2] No such file or directory
program finished with exit code 1
elapsedTime=0.005613

It doesn't like the new invocation that resulted from the changes in #19806.

[highfive]

cc @aneeshusa