log: don't override default filters (#1504)

[jdjaustin]

Summary of changes
Changes introduced in this pull request:

• don't override default filters when RUST_LOG environment variable is set

Reference issue to close (if applicable)

Closes #1504 .

Not sure if there is a default test suite; please advise best way to test if possible.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1530 (0c21654) into main (0c5ab30) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1530      +/-   ##
==========================================
- Coverage   44.42%   44.41%   -0.01%     
==========================================
  Files         310      310              
  Lines       28112    28112              
==========================================
- Hits        12488    12487       -1     
- Misses      15624    15625       +1     

Impacted Files	Coverage Δ
forest/src/logger/mod.rs	0.00% <0.00%> (ø)
vm/actor/src/builtin/miner/mod.rs	33.49% <0.00%> (-0.05%)	⬇️

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[LesnyRumcajs]

Looks good, quite useful first PR!