fix: prevent stdout duplication when capture_stdout is enabled

[cyril23]

Summary

Fix output duplication bug when capture_stdout: true is enabled.

Fixes #397

Problem

The bug was introduced in PR #374 (commit b6690ee) during a refactoring. The original implementation was correct:

# Original - correct
echo 'stdout<<EOF' >>$GITHUB_OUTPUT
sh -c "${TARGET} $*" | tee -a $GITHUB_OUTPUT
echo 'EOF' >>$GITHUB_OUTPUT

The refactoring wrapped everything in a subshell but kept tee -a:

# After refactoring - buggy
if [[ "${INPUT_CAPTURE_STDOUT}" == 'true' ]]; then
  {
    echo 'stdout<<EOF'
    "${TARGET}" "$@" | tee -a "${GITHUB_OUTPUT}"  # <-- 1st write
    echo 'EOF'
  } >>"${GITHUB_OUTPUT}"  # <-- 2nd write (subshell redirect)

This causes command output to be written twice to GITHUB_OUTPUT:

1. Via tee -a "${GITHUB_OUTPUT}" directly
2. Via the subshell { ... } >>"${GITHUB_OUTPUT}" redirect

Solution

Change tee -a "${GITHUB_OUTPUT}" to tee /dev/stderr:

• Real-time output is still visible in the Actions log (via stderr)
• Output is only written once to GITHUB_OUTPUT (via the subshell redirect)

Testing

Tested in production CI/CD pipelines deploying to multiple servers. Before the fix, all captured output lines appeared duplicated. After the fix, output appears correctly once.

[appleboy]

@cyril23 I will take it.