Fix race in nack responder

[aalekseevx]

Description

Fixes the following panic:

fatal error: concurrent map read and map write 
github.com/pion/interceptor/pkg/nack.(*ResponderInterceptor).BindLocalStream.func1(0xc0b16fae10, {0xc03af93210, 0x467, 0x467}, 0xc0a8afe9c0)
github.com/pion/interceptor/pkg/nack/responder_interceptor.go:126 +0x145 github.com/pion/interceptor.RTPWriterFunc.Write

Looks like race was introduced in d34a3c5

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.41%. Comparing base (60fb984) to head (3755e63).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #311   +/-   ##
=======================================
  Coverage   71.41%   71.41%           
=======================================
  Files          79       79           
  Lines        4530     4531    +1     
=======================================
+ Hits         3235     3236    +1     
  Misses       1158     1158           
  Partials      137      137           

Flag	Coverage Δ
go	71.41% <100.00%> (+0.13%)	⬆️
wasm	69.38% <100.00%> (-0.13%)	⬇️

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[aalekseevx]

Thanks, @nils-ohlmeier