Terraform workflow improvements

[itegulov]

Having an environment deployed per PR is wasteful as many of them don't need it. Let's figure out a way to do this conditionally; so far I am considering the following options:

1. Wait until the PR author posts a comment, e.g. terraform deploy. See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issue_comment
2. Only do this for PRs that contain [deploy] in the title. For example, feat[deploy]: new ui will deploy enviornment, while fix: README won't
3. Make deploys purely manual. The author will have to go to the Github Action tab, press a button there and enter the PR number.

Tasks

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1. Stop running dev flow for PRs that don't target develop branch
2. Make feature enviornments conditional
3. Make workflows wait for lock in case they are being run in parallel
4. Delete feature env workspace after its contents are destroyed
5. (Optional) figure out if it is possible to make terraform plan ignore that the currently deployed Docker image is not present on the runner