Ability to specify pg_config from env variables

[var77]

This change will help us to specify the pg_config from env variable PG_CONFIG.
This is needed for homebrew formula, so we will make sure that we are compiling against the same version that we have found in homebrew.

[github-actions]

Benchmarks

metric	old	new	pct change
recall (after create)	0.740	0.740	-
recall (after insert)	0.738	0.774	+4.88%
select bulk tps	214.972	218.022	+1.42%
select bulk latency (ms)	36.515	34.106	-6.60%
select bulk latency (stddev ms)	8.139	14.026	+72.33%
create latency (ms)	1497.447	1603.106	+7.06%
insert bulk tps	14.138	13.407	-5.17%
insert bulk latency (ms)	70.718	74.575	+5.45%
insert bulk latency (stddev ms)	3.203	2.949	-7.93%
disk usage (bytes)	6348800.000	6348800.000	-

[codecov]

Codecov Report

Merging #182 (d4fdaba) into main (39b5b00) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #182   +/-   ##
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  Coverage   83.29%   83.29%           
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  Files          17       17           
  Lines        1161     1161           
  Branches      245      245           
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  Hits          967      967           
  Misses         85       85           
  Partials      109      109