Bump mamba-org/provision-with-micromamba from 14 to 15

Bumps [mamba-org/provision-with-micromamba](https://github.com/mamba-org/provision-with-micromamba) from 14 to 15.
- [Release notes](https://github.com/mamba-org/provision-with-micromamba/releases)
- [Commits](mamba-org/provision-with-micromamba@v14...v15)

---
updated-dependencies:
- dependency-name: mamba-org/provision-with-micromamba
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/nightlies.yml

@@ -60,7 +60,7 @@ jobs:
 
         # N.B. anaconda-client is only maintained on the main channel
       - name: Install anaconda-client
-        uses: mamba-org/provision-with-micromamba@v14
+        uses: mamba-org/provision-with-micromamba@v15
         with:
           environment-file: false
           environment-name: nightlies