I'm creating scripts for deployment, so it was simpler to use xp_regwrite:

EXEC master..xp_regwrite

@rootkey='HKEY_LOCAL_MACHINE',

@key='SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\SQLServerAgent',

@value_name='MsxEncryptChannelOptions',

@type='REG_SZ',

@value='00000000'

-- 0 Disables encryption between this target server and the master server. her means.

-- 1 Enables encryption only between this target server and the master server, but no certificate validation is required.

-- 2 Default. Enables full SSL encryption and certificate validation between this target server and the master server.