remove usage of getenv from codebase

[kevinpapst]

Description

BC break: removed registration of .env with putenv() - do not rely on getenv() as it is not thread-safe

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #1861 into master will increase coverage by 0.00%.
The diff coverage is 78.57%.

@@            Coverage Diff            @@
##             master    #1861   +/-   ##
=========================================
  Coverage     92.83%   92.84%           
- Complexity     5164     5169    +5     
=========================================
  Files           489      489           
  Lines         15989    15990    +1     
=========================================
+ Hits          14844    14846    +2     
+ Misses         1145     1144    -1     

Impacted Files	Coverage Δ	Complexity Δ
src/Controller/DoctorController.php	72.38% <72.72%> (+0.95%)	37.00 <3.00> (-1.00)	⬆️
...endencyInjection/Compiler/DoctrineCompilerPass.php	76.74% <75.00%> (-1.04%)	20.00 <0.00> (+6.00)	⬇️
src/Configuration/MailConfiguration.php	100.00% <100.00%> (ø)	4.00 <4.00> (ø)