fix: check return value of os.MkdirAll

[G4Vi]

Fixes #112

Before when ran without sudo, extism lib install ignored errors when creating directories:

patrick@Mac local % extism lib install
Installing v1.12.0 to /usr/local
Fetching https://github.com/extism/extism/releases/download/v1.12.0/libextism-aarch64-apple-darwin-v1.12.0.tar.gz
Error: open /usr/local/include/extism.h: no such file or directory

Now:

gavin@Gavins-MacBook-Pro cli % go run extism/main.go lib install 
Installing v1.12.0 to /usr/local
Fetching https://github.com/extism/extism/releases/download/v1.12.0/libextism-aarch64-apple-darwin-v1.12.0.tar.gz
Error: mkdir /usr/local/include: permission denied