Reduce length of md5 in scss caching

[skjnldsv]

Easier to debug.

Before	After

[rullzer]

fine by me

[MorrisJobke]

Unit test failures.

[codecov]

Codecov Report

Merging #7504 into master will decrease coverage by 16.39%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #7504      +/-   ##
============================================
- Coverage     51.16%   34.77%   -16.4%     
  Complexity    24865    24865              
============================================
  Files          1601     1601              
  Lines         94683    94683              
  Branches       1368     1368              
============================================
- Hits          48446    32923   -15523     
- Misses        46237    61760   +15523

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Template/SCSSCacher.php	6.34% <0%> (-63.5%)	36 <0> (ø)
lib/private/Files/ObjectStore/Mapper.php	0% <0%> (-100%)	2% <0%> (ø)
lib/private/App/AppStore/Bundles/Bundle.php	0% <0%> (-100%)	2% <0%> (ø)
...ddleware/Security/Exceptions/NotAdminException.php	0% <0%> (-100%)	1% <0%> (ø)
...Check/Iterator/ExcludeFileByNameFilterIterator.php	0% <0%> (-100%)	2% <0%> (ø)
apps/files_trashbin/appinfo/app.php	0% <0%> (-100%)	0% <0%> (ø)
apps/dav/lib/CalDAV/Activity/Filter/Calendar.php	0% <0%> (-100%)	7% <0%> (ø)
lib/private/App/CodeChecker/EmptyCheck.php	0% <0%> (-100%)	6% <0%> (ø)
lib/public/AppFramework/Http/DownloadResponse.php	0% <0%> (-100%)	1% <0%> (ø)
apps/user_ldap/lib/Settings/Section.php	0% <0%> (-100%)	5% <0%> (ø)
... and 515 more

[skjnldsv]

Test failure is unrelated, merging.