feat(ci): enable manual trigger of preview CI

[tthvo]

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Related to #1045

Description of the change:

Allow manual trigger for preview CI.

Motivation for the change:

Useful when we need a rebuild without changes source files.

How to tests

Push to fork and go to Action tab.

[github-actions]

Test image available:

CRYOSTAT_IMAGE=ghcr.io/cryostatio/cryostat-web:pr-1046-0b2dd8228d531c892eec7ffe79a64c2a399134e3 sh smoketest.sh