utils: add CMake to Linux checkout

[compnerd]

In order to prepare for a migration to CMake 3.15 to enable the use of
the new Swift support, we will add a build of CMake for Linux to ease
the transition. This adds CMake to the checkout on Linux.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @shahmishal @spevans @gottesmm @jrose-apple

[compnerd]

@swift-ci please smoke test

[spevans]

Should this only checkout the cmake repo if the installed version of cmake < 3.15? I assume we will need a check like that somewhere to avoid building it as well if it doesn't need to be.

[compnerd]

I don't think it really matters if we checkout the sources, its not a large repository and doesn't take very long to clone. I don't see such a check for clones on any of the other repositories. You are correct that we will need the check though for the build.

[shahmishal]

This looks good to me, however when we enable building cmake in the build-script we should check if the dictionary exist before trying to build cmake. Otherwise we should fall back to using the system cmake.