fix(share/eds): fix test flake

[Wondertan]

Caught a flake, and here is a fix: init map on Start, rather than in gc routine

 --- FAIL: TestEDSStore_GC (0.01s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1011ad8]

goroutine 15855 [running]:
testing.tRunner.func1.2({0x110b7a0, 0x1d73c30})
	/opt/hostedtoolcache/go/1.20.4/x64/src/testing/testing.go:1526 +0x24e
testing.tRunner.func1()
	/opt/hostedtoolcache/go/1.20.4/x64/src/testing/testing.go:1529 +0x39f
panic({0x110b7a0, 0x1d73c30})
	/opt/hostedtoolcache/go/1.20.4/x64/src/runtime/panic.go:884 +0x213
github.com/celestiaorg/celestia-node/share/eds.TestEDSStore_GC(0xc01f719860)
	/home/runner/work/celestia-node/celestia-node/share/eds/store_test.go:178 +0x258
testing.tRunner(0xc01f719860, 0x13c1488)
	/opt/hostedtoolcache/go/1.20.4/x64/src/testing/testing.go:1576 +0x10b
created by testing.(*T).Run
	/opt/hostedtoolcache/go/1.20.4/x64/src/testing/testing.go:1629 +0x3ea

[codecov-commenter]

Codecov Report

Merging #2364 (d8febe4) into main (c41738a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2364      +/-   ##
==========================================
+ Coverage   50.84%   50.86%   +0.02%     
==========================================
  Files         154      154              
  Lines        9746     9746              
==========================================
+ Hits         4955     4957       +2     
+ Misses       4355     4353       -2     
  Partials      436      436              

Impacted Files	Coverage Δ
share/eds/store.go	60.60% <100.00%> (ø)

... and 4 files with indirect coverage changes