fix a line of code which causes CodecLibz build to fail

[kindlychung]

Got this error while building CodecLibz:

│ ERROR: LoadError: LibraryProduct(nothing, ["libz"], :libz, "Prefix(/home/kaiyin/.julia/packages/CodecZlib/hJvUf/deps/usr)") is not satisfied, cannot generate deps.jl!

This commit fixes the problem.

[hhaensel]

Hi @kindlychung ,
the code that you propose is executed if you call write_deps_file with isolate=false.
So if you want to correct the behaviour of BinaryProvider for yourself, better use

    verbose::Bool=false, isolate::Bool=false) where {P <: Product}

in line 398 of Products.jl
The reason, why the dlopen-check with isolate=true fails is not yet understood, but probably resolvable by deleting .julia
The ìsolate option was introduced by @staticfloat , I think, to have a more robust checking of the executability (is that an English word ;-) ) of a library product.