R has no per-user script directory to complement R_LIBS_USER.
So packages have no option but to install scripts to $R_HOME/bin.
With the `r` Apt package since 4.0, regular users don't have permission
to write there.

The commit allows the current user to create new executables there while
disallowing messing with existing ones.

• Commit b98f945
• Compare b98f945dddf4
• Branch r_script_allow

native-api