build-ffmpeg: ffmpeg source and cmake minor version update

[rokibhasansagar]

• The previous script was breaking in the middle of compilation without this patch. Now the script will download cmake source from proper path.
• Version number fetching could be made autonomous if jq could be used in all build platforms (IDK if they are prebuilt in the systems) as cmake keeps note of the latest version in a JSON file in the Release Directory.

[mxsrm]

Using "https://github.com/Kitware/CMake/releases/download/v3.21.0/cmake-3.21.0.tar.gz" instead of the path from cmake.org will prevent this from breaking every time a new version is released.

[rokibhasansagar]

Using "https://github.com/Kitware/CMake/releases/download/v3.21.0/cmake-3.21.0.tar.gz" instead of the path from cmake.org will prevent this from breaking every time a new version is released.

I updated the version number manually, and your suggestion does the exact thing.
There's no Automation in either of our methods 👎🏼

[mxsrm]

Yes but when cmake updates to a new version the link at https://cmake.org/files/LatestRelease/ is deleted. This script fails. My method will continue to work because the github link stays valid and the build will continue to work with the (now old) version of cmake in the script.