Only run slash-command-dispatch workflow on pull requests

[jeskew]

I'm getting an email about a failed workflow every time I comment on an issue. This PR adds a condition to slashCommandDispatch in line with the example on https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issue_comment-on-issues-only-or-pull-requests-only

Microsoft Reviewers: Open in CodeFlow

[github-actions]

Test this change out locally with the following install scripts (Action run 8254992995)

VSCode

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-vsix.sh) --run-id 8254992995

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-vsix.ps1) } -RunId 8254992995"

Azure CLI

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-cli.sh) --run-id 8254992995

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-cli.ps1) } -RunId 8254992995"

[github-actions]

Test Results

    66 files   -     33      66 suites   - 33   22m 6s ⏱️ - 19m 6s
10 680 tests  -     20  10 678 ✅  -     20  2 💤 ±0  0 ❌ ±0 
25 250 runs   - 12 621  25 246 ✅  - 12 619  4 💤  - 2  0 ❌ ±0 

Results for commit 442fdc0. ± Comparison against base commit 5e20b29.