Patch un-imported `Accessors` in `MCMCChainsExt` #976

Job	Run time
Julia 1 on ubuntu-latest (x64)	14m 3s
Julia 1 on macOS-latest (x64)	12m 44s
Julia 1 on windows-latest (x64)	14m 35s
Julia 1.9 on ubuntu-latest (x64)	28m 17s
Julia 1.9 on macOS-latest (x64)	26m 57s
Julia 1.9 on windows-latest (x64)	27m 7s
Julia nightly on ubuntu-latest (x64)	17m 27s
Julia nightly on macOS-latest (x64)	18m 49s
Julia nightly on windows-latest (x64)	19m 42s
	2h 59m 41s

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