use 'os-latest` in ngen ci

[madMatchstick]

Workflow ngen_integration specifically calls out mac & ubuntu versions; lets update to -latest.

Fixes issue #49

[PhilMiller]

Ack, I wish I'd seen this a bit faster. I would have recommended specifying ubuntu-22.04 and keeping macos-12. The challenge with using -latest is that toolchain and other configuration changes can appear unexpectedly, when the first PR gets pushed after Github makes an update, and fail for unrelated reasons. When we're being relatively strict about compiler warnings that can change from one compiler version to another, that potentially causes a lot of turmoil.

[madMatchstick]

@PhilMiller - This one is easily revert-able. I've noticed that many of the other formulation ngen_integration workflows use -latest which is why I brought it about. We can definitely work on specifying ubuntu and mac versions here on out... -thanks!