Use consistent template variable names

[nacl]

rules_pkg uses multiple template variables throughout the codebase, including:

• %VAR%
• {VAR}
• ${VAR}

Pick one (not necessarily from these), and make all uses consistent where possible. It may not be in some locations (like in the pkg_rpm script.

_Originally posted by @aiuto in https://github.com/bazelbuild/rules_pkg/pull/380#discussion_r670118887_

[aiuto]

Considerations for evaluating a choice:

• are we trying to match the style of a language?
  • just because Starlark looks like Python, does that mean python syntax is right?
  • just because Bazel is written in Java, and the Bazel developers sometimes use Java style, does that mean that is right?
  • probably neither matters because the users are coming from other build tools and probably use neither language on a daily basis.
• are we trying to match the style of a well known build tool?
  • this seems reasonable if we consider that people are often migrating from something to Bazel
  • autotools style? (%{foo})
  • Make variables. ($(foo))
  • CMake (${FOO})? Maven?
  • bash style (and which one, $FOO or ${FOO}?)
  • Windows .bat style (%FOO%). Why not? There are more windows programmers than *ix people sometimes give credit for.

[aiuto]

My current vote is that python style will be the least surprising, because Starlark looks so much like Python, that carrying over idiomatic coding styles is more likely to be a win than matching another build tool style. Thus my vote is to unify around {var}.

[aiuto]

Resolved. We will use {var}