Bump core in local Makefile to 20.1.0 #593

Job	Run time
amd64 / build-stellar-friendbot	48s
amd64 / build-rs-stellar-xdr	1m 44s
amd64 / build-stellar-horizon	2m 29s
amd64 / build-stellar-core	30s
amd64 / build-stellar-soroban-rpc	2m 11s
arm64 / build-rs-stellar-xdr	2m 15s
arm64 / build-stellar-core	22s
arm64 / build-stellar-friendbot	4m 32s
arm64 / build-stellar-horizon	8m 8s
arm64 / build-stellar-soroban-rpc	1m 53s
amd64 / build	4m 15s
amd64 / test (pubnet)	1m 24s
amd64 / test (local)	1m 37s
amd64 / test (local, --enable-soroban-rpc)	1m 53s
amd64 / push-pr	0s
arm64 / build	7m 38s
amd64 / push-release	0s
arm64 / test (pubnet)	1m 12s
arm64 / test (local)	2m 29s
arm64 / test (local, --enable-soroban-rpc)	2m 52s
arm64 / push-pr	0s
arm64 / push-release	0s
manifest / push	0s
complete	1s
	48m 13s

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