Add `concurrency` block to cancel previous CI runs

[iofq]

Resolves #889

[iofq]

Example from a dummy commit to my fork:

https://github.com/iofq/celestia-node/actions

Jobs were cancelled as intended. Workflow name is <workflow>-<ref/PR>, in this case it wasn't a PR so we just have refs/heads/<branch>.

One consideration here is Github is spamming me with workflow cancellation emails, so maintainers would likely want to adjust Github notification settings or setup some sort of email filtering.

[codecov-commenter]

Codecov Report

Merging #894 (4468b05) into main (ace2fda) will decrease coverage by 0.54%. The diff coverage is 39.34%.

@@            Coverage Diff             @@
##             main     #894      +/-   ##
==========================================
- Coverage   58.03%   57.48%   -0.55%     
==========================================
  Files         132      133       +1     
  Lines        7937     8268     +331     
==========================================
+ Hits         4606     4753     +147     
- Misses       2855     3040     +185     
+ Partials      476      475       -1     

Impacted Files	Coverage Δ
cmd/celestia/full.go	40.98% <0.00%> (-1.39%)	:arrow_down:
cmd/celestia/light.go	40.98% <0.00%> (-1.39%)	:arrow_down:
cmd/flags_header.go	46.42% <0.00%> (ø)
cmd/start.go	30.76% <0.00%> (+1.50%)	:arrow_up:
core/fetcher.go	51.78% <ø> (ø)
ipld/get.go	82.60% <ø> (+5.46%)	:arrow_up:
ipld/get_shares.go	90.90% <ø> (-0.93%)	:arrow_down:
ipld/helpers.go	100.00% <ø> (ø)
node/state/keyring.go	1.96% <0.00%> (ø)
service/rpc/state.go	0.00% <0.00%> (ø)
... and 33 more

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[Wondertan]

@iofq, hello, thank you for this PR, and sorry for taking too much time to respond to it. I am super happy to land this. We still need to resolve the comment Jonh made though

[adlerjohn]

Thanks for working through this @iofq! Good work!