Fix ambiguous call build break in Fuzzing project

[MihaZupan]

Compiler isn't happy about

The call is ambiguous between the following methods or properties: 'Assert.SequenceEqual(Span, Span)' and 'Assert.SequenceEqual(ReadOnlySpan, ReadOnlySpan)'

here

runtime/src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/Base64Fuzzer.cs

Line 142 in 144dcae

Assert.SequenceEqual(input, decoded);

where the args are Span<byte> and byte[].

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[stephentoub]

cc: @jjonescz, @333fred