Control log file name via environment variable

[aalhossary]

Using PLUMED_LOG_FILE environment variable to control the log file name instead of being hardcoded to PLUMED.OUT

Description

Quick & Dirty way to control the log file name instead of being hardcoded to PLUMED.OUT is to use PLUMED_LOG_FILE env variable.
If the variable is set, use it, otherwise, use the default value.

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[aalhossary]

When approved, should target #1413