Actions: Add a switch to ignore coveralls upload failures

Coveralls is down this morning: https://status.coveralls.io/

```
Update - Outage will be extended for at least several more hours. To avoid further disruption to your CI workflows, we recommend employing the fail-on-error: false input option available with all official coveralls integrations. See: https://docs.coveralls.io/integrations#official-integrations.
Feb 24, 2026 - 19:28 PST
```

This changes adds a switch to enable/disable coveralls failures.

This should be set back to TRUE when coveralls is working again.

Author: simmsa

.github/workflows/main.yml

@@ -10,6 +10,12 @@ on:
       - main
       - develop
 
+# Coveralls coverage reporting toggle.
+# Set to true when coveralls is healthy; false during outages.
+# Check status: https://status.coveralls.io/
+env:
+  COVERALLS_FAIL_ON_ERROR: false
+
 jobs:
   # Only PRs to main run on all OSes. Everything else (PRs to
   # develop, pushes) runs on Ubuntu only for faster feedback.
@@ -270,6 +276,7 @@ jobs:
           flag-name: conda-${{ runner.os }}-py${{ matrix.python-version }}
           parallel: true
           path-to-lcov: ./coverage.lcov
+          fail-on-error: ${{ env.COVERALLS_FAIL_ON_ERROR }}
 
   conda-forge-build:
     name: conda-forge-${{ matrix.os }}/${{ matrix.python-version }}
@@ -334,6 +341,7 @@ jobs:
           flag-name: conda-forge-${{ runner.os }}-py${{ matrix.python-version }}
           parallel: true
           path-to-lcov: ./coverage.lcov
+          fail-on-error: ${{ env.COVERALLS_FAIL_ON_ERROR }}
 
   pip-build:
     name: pip-${{ matrix.os }}/${{ matrix.python-version }}
@@ -391,6 +399,7 @@ jobs:
           flag-name: pip-${{ runner.os }}-py${{ matrix.python-version }}
           parallel: true
           path-to-lcov: ./coverage.lcov
+          fail-on-error: ${{ env.COVERALLS_FAIL_ON_ERROR }}
 
   hindcast-calls:
     name: hindcast-${{ matrix.os }}/${{ matrix.python-version }}
@@ -475,6 +484,7 @@ jobs:
           flag-name: hindcast-${{ runner.os }}-py${{ matrix.python-version }}
           parallel: true
           path-to-lcov: ./coverage.lcov
+          fail-on-error: ${{ env.COVERALLS_FAIL_ON_ERROR }}
 
   test-wheel-packaging:
     name: Test Built Wheel