fix ENC 'echo' script template to prevent expansion

Currently the template script for creating the 'echo' script used for
ENC data wraps the data provided by the actual ENC in `cat <<-EOF`,
which causes the enclosed data to be variable-expanded, escape
characters (like double backslashes) to be replaced with their
concrete representations, etc.

This breaks things horribly if you have (e.g.) JSON data with
escaped strings (`"C:\\Windows"`), which will be processed and
output without those escapes (`"C:\Windows"`), which is invalid,
and definitely not what the user intended under any circumstance.

Using the 'quoted' variant of shell heredocs (`cat <<-'EOF'`) stops
this expansion.

lib/octocatalog-diff/catalog-util/builddir.rb

@@ -207,7 +207,7 @@ def install_enc(logger)
         enc_path = File.join(@tempdir, 'enc.sh')
         File.open(enc_path, 'w') do |f|
           f.write "#!/bin/sh\n"
-          f.write "cat <<-EOF\n"
+          f.write "cat <<-'EOF'\n"
           f.write enc_obj.content
           f.write "\nEOF\n"
         end

spec/octocatalog-diff/tests/catalog-util/builddir_spec.rb

@@ -796,7 +796,7 @@
         testobj = OctocatalogDiff::CatalogUtil::BuildDir.new(options, logger)
         enc = File.join(testobj.tempdir, 'enc.sh')
         expect(File.file?(enc)).to eq(true)
-        expect(File.read(enc)).to eq("#!/bin/sh\ncat <<-EOF\n---\n\nEOF\n")
+        expect(File.read(enc)).to eq("#!/bin/sh\ncat <<-'EOF'\n---\n\nEOF\n")
       end
     end