ci: avoid triggered-by-schedule and triggered-by-merge builds cancelling each other

[jameslamb]

Contributes to rapidsai/build-planning#278

Today, merging a PR while the nightly build is in progress can lead to it being cancelled.
This updates CI configurations to prevent that.

[jameslamb]

hmmmm I'm still seeing failures like this:

The following tests FAILED:
	1184 - offline-build-makefile (Failed)                   examples makefile
	1186 - offline-build-ninja (Failed)                      examples ninja
	1188 - offline-build-ninja_multi-config (Failed)         examples ninja_multi-config
	1190 - pin-dependencies-makefile (Failed)                examples makefile
	1192 - pin-dependencies-ninja (Failed)                   examples ninja
	1194 - pin-dependencies-ninja_multi-config (Failed)      examples ninja_multi-config
...
    /__w/rapids-cmake/rapids-cmake/build/examples/pin-dependencies-makefile-build/_deps/cccl-src/lib/cmake/libcudacxx/../../../libcudacxx/include/cuda/std/__cccl/compiler.h:184:30: error: The <cuda/stream_ref> header is deprecated and will be removed in the next major release. The contents were moved to <cuda/stream>. To ignore this message, define CCCL_IGNORE_DEPRECATED_STREAM_REF_HEADER. [-Werror]
      184 | #  define _CCCL_PRAGMA(_ARG) _Pragma(_CCCL_TO_STRING(_ARG))
          |                              ^~~~~~~

(build link)

I think this is what rapidsai/rmm#2513 was intended to fix. Looking into it.

[jameslamb]

/merge