Keep only needed unwrap statements

[tdejager]

While looking through the code I've noticed a whole bunch of unwrap statements. Would be good to get a close look into which of these could be replaced.

See:

$ rg unwrap -c

src/used_variables.rs:3
src/windows/link.rs:1
src/main.rs:10
src/windows/env.rs:2
src/recipe.rs:3
src/packaging.rs:11
src/post.rs:3
src/index.rs:25
src/variant_config.rs:14
src/unix/env.rs:2
src/linux/link.rs:1
src/linux/env.rs:8
src/metadata.rs:2
src/render/pin.rs:15
src/render/resolved_dependencies.rs:5
src/recipe/parser.rs:3
src/source/url_source.rs:6
src/recipe/stage1.rs:5
src/recipe/jinja.rs:3
src/recipe/error.rs:3
src/render/solver.rs:13
src/recipe/parser/build.rs:3
src/recipe/parser/about.rs:2
src/source/git_source.rs:13
src/source/mod.rs:16
src/recipe/parser/package.rs:2
src/macos/link.rs:1
src/recipe/parser/source.rs:1
src/test.rs:20
src/selectors.rs:19
src/env_vars.rs:7

[swarnimarun]

I have reviewed most of the code as well recently, can say largely most of it can be removed, some more easily than others, perhaps anyone making a PR consider doing it one file at a time.

[wolfv]

I am also happy to take on some of these, maybe during the next train ride or so. Will ping here which files I am tackling to avoid double work.

[wolfv]

I cleaned a few. Just to note, some are also unwrap_or_default and similar (you might want to use unwrap() as search).