ci: trigger mega-linter workflow also on pull requests

[jklare]

• so far, the linter was only running on push, which was fine for all internal PRs
• to also lint code from PRs that were not pushed to the repo directly, we need to enable the pipeline also for those

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	3		0	0	0.03s
✅ JSON	jsonlint	4		0	0	0.08s
✅ JSON	prettier	4		0	0	0.35s
✅ JSON	v8r	4		0	0	6.85s
✅ MARKDOWN	markdownlint	154		0	0	2.21s
✅ MARKDOWN	markdown-table-formatter	154		0	0	0.41s
✅ REPOSITORY	checkov	yes		no	no	17.17s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	1.69s
✅ REPOSITORY	trufflehog	yes		no	no	4.08s
✅ SPELL	codespell	162		0	0	0.5s
✅ SPELL	lychee	162		0	0	37.24s
✅ YAML	prettier	4		0	0	0.43s
✅ YAML	v8r	4		0	0	5.38s
✅ YAML	yamllint	4		0	0	0.5s

See detailed reports in MegaLinter artifacts

Your project could benefit from a custom flavor, which would allow you to run only the linters you need, and thus improve runtime performances. (Skip this info by defining FLAVOR_SUGGESTIONS: false)

• Documentation: Custom Flavors
• Command: npx mega-linter-runner@9.4.0 --custom-flavor-setup --custom-flavor-linters ACTION_ACTIONLINT,JSON_JSONLINT,JSON_V8R,JSON_PRETTIER,MARKDOWN_MARKDOWNLINT,MARKDOWN_MARKDOWN_TABLE_FORMATTER,REPOSITORY_CHECKOV,REPOSITORY_GIT_DIFF,REPOSITORY_SECRETLINT,REPOSITORY_TRUFFLEHOG,SPELL_LYCHEE,SPELL_CODESPELL,YAML_PRETTIER,YAML_YAMLLINT,YAML_V8R

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