build: Fix permissions of Preview GitHub action

[vorburger]

re. EndBug/pages-preview#20

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ ACTION	actionlint	1		0	0.05s
✅ COPYPASTE	jscpd	yes		no	1.85s
✅ EDITORCONFIG	editorconfig-checker	1		0	0.02s
✅ REPOSITORY	checkov	yes		no	12.72s
✅ REPOSITORY	gitleaks	yes		no	0.23s
✅ REPOSITORY	git_diff	yes		no	0.02s
✅ REPOSITORY	grype	yes		no	18.74s
✅ REPOSITORY	kics	yes		no	38.57s
✅ REPOSITORY	secretlint	yes		no	0.91s
✅ REPOSITORY	trivy	yes		no	6.51s
✅ REPOSITORY	trivy-sbom	yes		no	1.54s
✅ REPOSITORY	trufflehog	yes		no	4.06s
✅ YAML	prettier	1	0	0	0.67s
✅ YAML	v8r	1		0	2.26s
✅ YAML	yamllint	1		0	0.53s

See detailed report in MegaLinter reports
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff

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