[ISSUE] [RFC] enable `XBPS_STRICT` by default

[mhmdanas <mhmdanas@users.noreply.github.com>]

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New issue by mhmdanas on void-packages repository

https://github.com/void-linux/void-packages/issues/45507

Description:
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When a `vsed` doesn't apply, they currently print a warning by default, and the build goes on. This can hide potential issues. `xbps-src` does have an `-s` flag, which in that case causes the build to stop at the failing vsed. This flag _is_ passed in the CI, but large PRs skip CI and so authors of those PRs may never notice the now-outdated `vsed`s (see e.g. https://github.com/void-linux/void-packages/issues/42441). Therefore, I propose enabling the "strict" `vsed` behavior by default, and would even go so far as to suggest removing the non-strict behavior, because I honestly don't see its point.