Install virtualenv python module.

.travis.yml

@@ -6,6 +6,7 @@ matrix:
     - sudo: false
       before_install:
         - curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain none -y
+        - pip install virtualenv
         - source ~/.profile
       script:
          - ./mach test-tidy --no-progress --all
@@ -20,6 +21,7 @@ matrix:
         - sudo add-apt-repository 'deb http://apt.llvm.org/precise/ llvm-toolchain-precise-3.9 main' -y
         - sudo apt-get update -q
         - sudo apt-get install clang-3.9 llvm-3.9 llvm-3.9-runtime libunwind8-dev -y
+        - pip install virtualenv
         - curl -L http://servo-deps.s3.amazonaws.com/gstreamer/gstreamer-1.14-x86_64-linux-gnu.20190213.tar.gz | tar xz
         - sed -i "s;prefix=/opt/gst;prefix=$PWD/gst;g" $PWD/gst/lib/pkgconfig/*.pc
         - export PKG_CONFIG_PATH=$PWD/gst/lib/pkgconfig

etc/taskcluster/docker/base.dockerfile

@@ -16,8 +16,10 @@ RUN \
         #
         # Running mach
         python \
-        virtualenv \
+        python-pip \
         #
         # Installing rustup and sccache (build dockerfile) or fetching build artifacts (run tasks)
-        curl
+        curl && \
+    # Running mach
+    pip install virtualenv