cmake: Avoid contaminating parent project's cache with BUILD_SHARED_LIBS

[hebasto]

The CMake cache is global in scope. Therefore, setting the standard cache variable BUILD_SHARED_LIBS can inadvertently affect the behavior of a parent project.

Consider configuring Bitcoin Core without explicit setting BUILD_SHARED_LIBS:

$ cmake -B build -DBUILD_KERNEL_LIB=ON

According to CMake’s documentation, this should configure libbitcoinkernel as STATIC.
However, that's not the case:

$ cmake --build build -t libbitcoinkernel
[143/143] Linking CXX shared library lib/libbitcoinkernel.so

This PR:

1. Sets the BUILD_SHARED_LIBS cache variable only when libsecp256k1 is the top-level project.
2. Uses the SECP256K1_DISABLE_SHARED cache variable only when libsecp256k1 is included as a subproject.

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Alternatively, the entire code block can be dropped. In that case:

1. The CMake-base build system will build a static library by default.
2. During integration into a parent project, the static library can be forced as demonstrated here.

[hebasto]

cc @theuni