CircleCI must install Python in order to build system libs. (#4128)

The docker image change in #4110 seems to mean that `python` is no
longer available in the Docker image by default, which means that
the build scripts in `./libs` no longer run successfully on CircleCI.

We did not notice this change because CircleCI usually restores
these from a cache. But if we do something that invalidates that
cache (like modifying the files in `./libs`) then CircleCI starts
failing.

This commit fixes the issue by explicily installing `python` as
part of the dependencies for the system lib builds. It also includes
a cosmetic change to `./libs/README.md` to clear the CircleCI cache
and test that the fix actually works.

.circleci/config.yml

@@ -98,7 +98,7 @@ commands:
   build-desktop-libs:
     steps:
       - run: sudo apt-get update
-      - run: sudo apt-get install tcl
+      - run: sudo apt-get install python tcl
       - run:
           name: Install NSS build system dependencies
           command: sudo apt-get install ninja-build gyp zlib1g-dev

libs/README.md

@@ -18,3 +18,4 @@ This directory builds the required libraries for iOS, Android and desktop platfo
 
 * Android: `TARGET_ARCHS=("x86" "x86_64" "arm64" "arm")`
 * iOS: `TARGET_ARCHS=("x86_64" "arm64")`
+