pathogen-repo-build: use eval to run input command

Using `eval` to run the command to avoid potential errors
as we don't want limit the block scalars for the run input.

If run input includes a newline at the end, the direct interpolation of
`${{ inputs.run }}` results in a syntax error¹:
```
line 11: syntax error near unexpected token `|&'
```

¹ https://github.com/nextstrain/seasonal-flu/actions/runs/5604334827/jobs/10252159727#step:7:327

.github/workflows/pathogen-repo-build.yaml

@@ -200,10 +200,13 @@ jobs:
           | tee -a "$GITHUB_ENV"
 
       - name: Run build via ${{ inputs.runtime }}
+        env:
+          NEXTSTRAIN_BUILD_COMMAND: ${{ inputs.run }}
         run: |
+          # shellcheck disable=SC2154
           set -x
 
-          ${{ inputs.run }} |& tee build.log
+          eval "$NEXTSTRAIN_BUILD_COMMAND" |& tee build.log
 
       - if: ${{ inputs.runtime == 'aws-batch' }}
         name: Get AWS Batch job id