change properties secrets env to SECRETS_FILE

[jetersen]

Fixes #1336

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[codecov]

Codecov Report

Merging #1435 into master will decrease coverage by 0.08%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #1435      +/-   ##
============================================
- Coverage     80.66%   80.58%   -0.09%     
+ Complexity      811      810       -1     
============================================
  Files            66       66              
  Lines          2333     2333              
  Branches        329      329              
============================================
- Hits           1882     1880       -2     
  Misses          351      351              
- Partials        100      102       +2     

Impacted Files	Coverage Δ	Complexity Δ
...gins/casc/impl/secrets/PropertiesSecretSource.java	81.25% <50.00%> (-6.25%)	7.00 <0.00> (ø)
.../plugins/casc/impl/secrets/DockerSecretSource.java	66.66% <0.00%> (-11.12%)	3.00% <0.00%> (-1.00%)