`prefix_paths` double-slashes absolute env-var paths

[moshe-elastix]

prefix_paths double-slashes absolute env-var paths

Summary

Environment variables in dependency path: fields do not work when the
variable expands to an absolute path. Bender prepends the consuming
Bender.yml's parent directory to the template string (e.g. ${ABS})
before substituting the env var, so the substitution happens inside an
already-prefixed path and produces a malformed result like
<parent>//<absolute-expansion>.

The prefix step should be skipped (or applied after expansion) when the
expansion produces an absolute path. As is, env vars in path: are
effectively unusable for absolute paths — direct absolute literals and
relative env vars both work, only absolute env-var expansions break.

Bender version

0.31.0 and master (0a98f9e).

Setup

/tmp/x/
├── a/Bender.yml
└── b/Bender.yml

a/Bender.yml:

package: { name: a }

b/Bender.yml:

package: { name: b }
dependencies:
  a: { path: "${ABS}" }

Repro

cd /tmp/x/b && ABS=/tmp/x/a bender update

Expected

a resolves to /tmp/x/a.

Actual

error[E32]: Path /tmp/x/b//tmp/x/a for dependency a does not exist.

Note the // between b/ and /tmp/x/a — bender concatenated the parent dir
onto the template, then expanded ${ABS} inside the combined string.