Provide a proper package ID to get rid of the following error:
`No receipt for 'radare2.pkg' found at '/'.`
Prepend a `/` for each returned path by the `pkgutil` command, to make
it an absolute path instead of a relative one.
Avoid invoking the `sudo` command for each file, which speeds up the
command significantly.
Avoid ambiguous signing certificates on macOS.
If a user manually installed a new code signing certificate into the
login keychain as documented, the install script will fail to sign r2
because the `sys/macos-cert.sh` script generates a new certificate in
the System keychain. This results in the following error when signing:
```
org.radare.radare2: ambiguous (matches "org.radare.radare2" in /Users/user/Library/Keychains/login.keychain-db and "org.radare.radare2" in /Library/Keychains/System.keychain)
```
To be constistent with `sys/macos-cert.sh`, change manual certificate
generation steps to install to System keychain instead of login
keychain.
Mention that code signing certificate generation is automated by the
install script before listing the manual steps to generate a new
code signing certificate.
