install: should show the same error regardless of whether target ends with slash

[cakebaker]

While reviewing #5686, I noticed the following difference between GNU install and uutils install:

// setup
$ touch a
$ mkdir target
$ touch target/b

// GNU install
$ install -t target/b -D a
install: failed to access 'target/b': Not a directory
$ install -t target/b/ -D a
install: failed to access 'target/b/': Not a directory

// uutils install
$ cargo run --features="unix" install -t target/b -D a
install: failed to access 'target/b': Not a directory
$ cargo run --features="unix" install -t target/b/ -D a
install: failed to create 'target/b/': entity already exists