Fix finding of compilers in CI pipelines

[msimberg]

Since https://github.com/spack/spack/pull/45784 we can no longer find nvhpc using spack compiler find. However, spack external find does work for nvhpc, and other compilers, so switching over to that everywhere.

[codacy-production[bot]]

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Coverage variation	Diff coverage
:white_check_mark: -0.05% (target: -1.00%)	:white_check_mark: ∅ (target: 90.00%)

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (88fae0f283cc6cf9d5e037f3d594447ef67a3cfd) | 18358 | 13721 | 74.74% | | | Head commit (acdba3515ad4e01e1d17ddd4b33401c46135253b) | 18358 (+0) | 13711 (-10) | 74.69% (**-0.05%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#1231) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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[msimberg]

https://github.com/spack/spack/pull/45784 is the PR that broke the build. Essentially it stops from finding NVHPC as a compiler, and thus concretization fails later. It's not clear to me why it doesn't find NVHPC anymore with that change.