Fix #409 Custom token rotation expiration time does not work if installation_store is passed at top level

[seratch]

This pull request fixes #409. The initialization for top level installation_store is a bit tricky ... (we may want to deprecate it in the future) but this works for sure.

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #436 (d82c206) into main (f70e1f4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #436   +/-   ##
=======================================
  Coverage   91.34%   91.34%           
=======================================
  Files         167      167           
  Lines        5497     5499    +2     
=======================================
+ Hits         5021     5023    +2     
  Misses        476      476           

Impacted Files	Coverage Δ
slack_bolt/app/app.py	87.58% <100.00%> (+0.02%)	⬆️
slack_bolt/app/async_app.py	94.16% <100.00%> (+0.01%)	⬆️

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