Prevent crash when libraries are unavailable during __init__

[AntonReinhard]

Changes the ENV[string] lookups to a get() with a default of "0". This prevents the ERROR: InitError: KeyError: key "JULIA_AMDGPU_CORE_MUST_LOAD" not found when running outside of the CI.

For example, previously I was getting

julia> using AMDGPU
┌ Warning: HSA runtime is unavailable, compilation and runtime functionality will be disabled.
└ @ AMDGPU ~/.julia/packages/AMDGPU/a1v0k/src/AMDGPU.jl:205
ERROR: InitError: KeyError: key "JULIA_AMDGPU_CORE_MUST_LOAD" not found
Stacktrace: ...

With this PR, I'm getting

julia> using AMDGPU
┌ Warning: HSA runtime is unavailable, compilation and runtime functionality will be disabled.
└ @ AMDGPU ~/.julia/packages/AMDGPU/zQy5G/src/AMDGPU.jl:205
...

julia> AMDGPU.functional()
false

as I would have expected for this case.