Fix IPFS export for recordings

[mjh1]

Previously we were assuming the source file video would always be available, but for recordings this doesn't get generated. To fix we now check for a static mp4 output as well as the video file (mp4s are always generated for recordings https://github.com/livepeer/task-runner/blob/d7580fa/task/upload.go#L525).

[codecov]

Codecov Report

Merging #201 (7f96194) into main (5bfc222) will decrease coverage by 0.05955%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##                main        #201         +/-   ##
===================================================
- Coverage   13.72549%   13.66594%   -0.05955%     
===================================================
  Files             18          18                 
  Lines           2295        2305         +10     
===================================================
  Hits             315         315                 
- Misses          1956        1966         +10     
  Partials          24          24                 

Impacted Files	Coverage Δ
task/export.go	0.00000% <0.00000%> (ø)

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Impacted Files	Coverage Δ
task/export.go	0.00000% <0.00000%> (ø)