granted: install shell wrappers and assumego symlink

The formula was only installing the granted binary. This left the
package non-functional because the assume workflow requires three
additional components that weren't being installed.

The assume script is a POSIX shell wrapper that calls assumego and
exports AWS credentials into the current shell environment. The
assume.fish script does the same for fish shell. The assumego binary
is accessed via symlink to the granted binary - the code checks argv[0]
to determine behavior.

Without these components, users couldn't actually assume AWS roles
because the credentials wouldn't propagate to their shell session.

This adds installation of scripts/assume and scripts/assume.fish from
the source tree, and creates the assumego -> granted symlink during
the install phase.

Author: chrnorm

Formula/g/granted.rb

@@ -20,6 +20,13 @@ class Granted < Formula
   def install
     ldflags = "-s -w -X github.com/common-fate/granted/internal/build.Version=#{version}"
     system "go", "build", *std_go_args(ldflags:), "./cmd/granted"
+
+    # Install shell wrapper scripts
+    bin.install "scripts/assume"
+    bin.install "scripts/assume.fish"
+
+    # Create assumego symlink
+    bin.install_symlink "granted" => "assumego"
   end
 
   test do