45615de26c ci: Fix default retry script usage (Hennadii Stepanov)
Pull request description:
On master (5352d14b37) `CI_RETRY_EXE=${CI_RETRY_EXE:retry}` works as a [Substring Expansion](https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html), and that is wrong.
If `CI_RETRY_EXE` variable was unset initially, its new value becomes an empty string, but not "retry" as one could expect. Consequently, the `${CI_RETRY_EXE} ...` command does _not_ use `ci/retry/retry` script.
This PR makes for `CI_RETRY_EXE` variable a usual parameter expansion, i.e., `${parameter:-word}`.
Reference: https://github.com/bitcoin/bitcoin/pull/18735#issuecomment-620095489
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