HostPlatform() is your current platform:
julia> HostPlatform()
Linux x86_64 {cxxstring_abi=cxx11, julia_version=1.6.4, libc=glibc, libgfortran_version=4.0.0, libstdcxx_version=3.4.29}
so you can have
platforms = [
HostPlatform()
]
but please do note that it contains some extra ABI specifications (like C++ string ABI and libgfortran version) that may be overly restrictive, in case you want to publish a JLL package.