ci: enable "run when submodule changes" with environment variables

We have a job in our CI (PR's x86_64-gnu-tools) that's supposed to run
only when a submodule is changed in the PR, and it works by having a
task at the start of the build that skips all the following tasks if the
condition isn't met.

Before this commit that task was gated with template parameters, which
is a unique feature of Azure Pipelines. To make our CI more generic this
commit switches the gate to use a simple environment variable plus a
condition, which should be supported on more CI providers.

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