Use cached app list

[MorrisJobke]

Followup to #7264

Before it just fetched the list again and again 🙈

[MorrisJobke]

I also added an unit tests that check this behaviour ;)

[skjnldsv]

Indeed! ^^

[codecov]

Codecov Report

Merging #7506 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #7506      +/-   ##
============================================
+ Coverage     51.11%   51.16%   +0.05%     
+ Complexity    24903    24865      -38     
============================================
  Files          1601     1601              
  Lines         94778    94683      -95     
  Branches       1368     1368              
============================================
+ Hits          48445    48446       +1     
+ Misses        46333    46237      -96

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Installer.php	57.64% <100%> (ø)	74 <0> (ø)	⬇️
lib/private/Server.php	80.9% <0%> (-0.12%)	134% <0%> (ø)
...rver_connector/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
...pps/encryption/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
...files_versions/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
apps/oauth2/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
.../files_sharing/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
...tenotification/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
apps/testing/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
apps/comments/composer/composer/autoload_real.php	0% <0%> (ø)	9% <0%> (-2%)	⬇️
... and 12 more