test: Change the installer for setup-matrix-synapse

[Hywan]

There is a bug in Synapse and changing the installer seems to fix the problem.

Trying to apply michaelkaye/setup-matrix-synapse#94 (comment) :-).

[Hywan]

@jplatte I'm asking a new review as I've replaced gnunicorn/setup-matrix-synapse by michaelkaye/setup-matrix-synapse in .github/workflows/coverage.yml.

[poljar]

I think now you're hitting: matrix-org/synapse#15858

[jplatte]

I wonder if we needed Ben's patch. In any case I think our serverName is now ineffective. This is what the forked action was doing differently: michaelkaye/setup-matrix-synapse#50

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (3180a1b) 76.72% compared to head (c955de8) 76.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2198      +/-   ##
==========================================
+ Coverage   76.72%   76.73%   +0.01%     
==========================================
  Files         164      164              
  Lines       17639    17640       +1     
==========================================
+ Hits        13533    13536       +3     
+ Misses       4106     4104       -2     

Impacted Files	Coverage Δ
crates/matrix-sdk-ui/src/room_list_service/room.rs	91.30% <ø> (ø)
...s/matrix-sdk-ui/src/room_list_service/room_list.rs	100.00% <ø> (ø)
crates/matrix-sdk-ui/src/room_list_service/mod.rs	96.51% <100.00%> (ø)
...rates/matrix-sdk-ui/src/room_list_service/state.rs	94.54% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[poljar]

Oh, it actually passed CI, nice.