Bump core in local Makefile to 20.1.0 #595

Job	Run time
amd64 / build-stellar-friendbot	49s
amd64 / build-stellar-horizon	2m 30s
amd64 / build-rs-stellar-xdr	1m 40s
amd64 / build-stellar-core	24s
amd64 / build-stellar-soroban-rpc	3m 12s
arm64 / build-rs-stellar-xdr	3m 15s
arm64 / build-stellar-core	25s
arm64 / build-stellar-friendbot	4m 31s
arm64 / build-stellar-horizon	8m 0s
arm64 / build-stellar-soroban-rpc	1m 58s
amd64 / build	4m 19s
amd64 / test (pubnet)	1m 22s
amd64 / test (local)	1m 40s
amd64 / test (local, --enable-soroban-rpc)	1m 44s
amd64 / push-pr	0s
arm64 / build	8m 49s
amd64 / push-release	0s
arm64 / test (pubnet)	1m 12s
arm64 / test (local)	2m 33s
arm64 / test (local, --enable-soroban-rpc)	2m 50s
arm64 / push-pr	0s
arm64 / push-release	0s
manifest / push	0s
complete	1s
	51m 14s

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