Optimizes AppendVec::scan_pubkeys() when using file io #7278

Job	Run time
check	3m 12s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	19m 28s
test (single-pool/cli, single-pool/program)	18m 51s
cargo-test-sbf (token/program)	8m 11s
test (token-upgrade/cli, token-upgrade/program)	16m 25s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	14m 30s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	14m 0s
cargo-test-sbf (token-upgrade/program)	8m 26s
cargo-test-sbf (feature-proposal/program)	8m 50s
cargo-test-sbf (governance/addin-mock/program, governance/program)	11m 30s
cargo-test-sbf (memo/program)	9m 0s
cargo-test-sbf (name-service/program)	8m 2s
cargo-test-sbf (stake-pool/program)	12m 29s
cargo-test-sbf (single-pool/program)	8m 49s
	2h 41m 43s

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