SVM: Reduce public visibility of sysvar_cache #6146

Job	Run time
check	3m 38s
cargo-test-sbf (token/program)	9m 12s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	14m 17s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	11m 37s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	17m 21s
cargo-test-sbf (token-upgrade/program)	9m 30s
test (single-pool/cli, single-pool/program)	16m 33s
test (token-upgrade/cli, token-upgrade/program)	16m 16s
cargo-test-sbf (feature-proposal/program)	10m 20s
cargo-test-sbf (governance/addin-mock/program, governance/program)	12m 57s
cargo-test-sbf (memo/program)	9m 43s
cargo-test-sbf (name-service/program)	8m 41s
cargo-test-sbf (stake-pool/program)	12m 47s
cargo-test-sbf (single-pool/program)	10m 4s
	2h 42m 56s

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