fix: nightly YAML multiline run command starts sbt without tasks

[He-Pin]

Summary

After #1030, the nightly job hangs for 5+ hours per matrix entry until canceled (e.g. run 25588927669).

The run: | literal block scalar turns each indented line into a separate shell command, so:

1. The first line sbt -Dpekko.http.parallelExecution=false -Dpekko.test.timefactor=2 runs sbt with no tasks and drops into the interactive REPL — the job hangs here until the workflow timeout.
2. The following -Dpekko.build.pekko.version=..., -DTLS_ENGINE_CONFIG=..., and "++ ..." mimaReportBinaryIssues test lines are dispatched as separate shell commands and never reach sbt.

The hung-job logs match exactly: sbt server started followed by long silence and [gc] Pause Full (System.gc()).

Fix

Switch the block scalar from | (literal, preserves newlines) to > (folded, joins lines with spaces) so the whole sbt invocation runs as a single command. Also drop the trailing whitespace on the first line.

run: >
  sbt -Dpekko.http.parallelExecution=false -Dpekko.test.timefactor=2
  -Dpekko.build.pekko.version=${{ matrix.PEKKO_VERSION }}
  -DTLS_ENGINE_CONFIG=${{ matrix.TLS_ENGINE_CONFIG }}
  "++ ${{ matrix.SCALA_VERSION }}" mimaReportBinaryIssues test

Verified locally — the YAML now parses to a single-line command:

sbt -Dpekko.http.parallelExecution=false -Dpekko.test.timefactor=2 -Dpekko.build.pekko.version=... -DTLS_ENGINE_CONFIG=... "++ ..." mimaReportBinaryIssues test

Test plan

• Wait for next nightly run (or trigger manually) and confirm jobs complete in normal time instead of timing out at 6h.

[pjfanning]

lgtm