pathogen-repo-build: use tee -a build.log`

I was confused that the CI workflow¹ did not have an AWS Batch Job
summary even though the tests have included an aws-batch runtime since
dab7abc.

The build.log from the build-output-aws-batch artifact² was incomplete
and did not include the AWS Batch Job ID. The build.log only includes
the last part of the build logs, so I suspect it's somehow overwriting
itself.

¹ https://github.com/nextstrain/.github/actions/runs/8209158885?pr=76
² https://github.com/nextstrain/.github/actions/runs/8209158885/artifacts/1310654964

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

@@ -276,7 +276,7 @@ jobs:
           # shellcheck disable=SC2154
           set -x
 
-          eval "unbuffer $NEXTSTRAIN_BUILD_COMMAND" |& tee build.log
+          eval "unbuffer $NEXTSTRAIN_BUILD_COMMAND" |& tee -a build.log
       # Attempt to get the AWS Batch ID even if the run build command failed
       # as long as the runtime is `aws-batch` and the `build.log` file exists
       - if: ${{ always() && inputs.runtime == 'aws-batch' && hashFiles('build.log') != '' }}

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

@@ -243,7 +243,7 @@ jobs:
           # shellcheck disable=SC2154
           set -x
 
-          eval "unbuffer $NEXTSTRAIN_BUILD_COMMAND" |& tee build.log
+          eval "unbuffer $NEXTSTRAIN_BUILD_COMMAND" |& tee -a build.log
 
       # Attempt to get the AWS Batch ID even if the run build command failed
       # as long as the runtime is `aws-batch` and the `build.log` file exists