Auto detect CPU cores in CI

[lilleyse]

Follow up from #124 (comment)

• Detects the number of logical CPU cores automatically using github-actions-cpu-cores. In a separate commit I echoed the count and got the expected values: 2 for linux and 8 for windows (only windows is using our AWS infrastructure).
• Upgraded all the other third party github actions

[kring]

Apparently even d:\cesium-omniverse\cesium-omniverse\.conan is too long for Conan builds to succeed. The only reason it seemed ok before was because the cache was populated successfully with a shorter path, and so it didn't need to build with the longer one. So I've commited a change to move it back to d:\.conan instead.

[weegeekps]