Allow peeking at cache using an env var

[jsoref]

Short description

codeql doesn't like unreachable code https://github.com/check-spelling-sandbox/pdns/security/quality/rules/py%2Funreachable-statement

Unreachable code - Code is unreachable

A simple way to make everyone happy (devs who want to peek, linters that believe no code should be dead) is to let devs set a var to peek.

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [x] read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• [ ] compiled this code
• [ ] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 19932359138

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 31 unchanged lines in 7 files lost coverage.
• Overall coverage remained the same at 73.119%

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Files with Coverage Reduction	New Missed Lines	%
pdns/recursordist/aggressive_nsec.cc	2	66.77%
pdns/dnsdistdist/dnsdist-carbon.cc	3	64.91%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/remote_logger.cc	3	58.62%
pdns/recursordist/syncres.cc	4	81.24%
pdns/misc.cc	8	61.45%
pdns/recursordist/test-syncres_cc1.cc	8	90.05%
<!--	Total:	31

Totals
Change from base Build 19931654036:	0.0%
Covered Lines:	128126
Relevant Lines:	164529

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💛 - Coveralls