Use `ureq` for "science" download, not `cargo install`'d `ptex`

[huonw]

This replaces an inline cargo install of https://github.com/a-scie/ptex with the ureq library. The ptex binary seems to only be used as a "portable" curl -O <url>, which can, I think, be done with code built into the package/ lib itself.

This change:

• conceptually simplifies the "build scie-pants" process
• makes it more reliable/secure (I think the cargo install wasn't using ptex's lockfile, because it didn't pass --locked)
• potentially speeds up the build when not cached (building ptex has to build a whole lot of libraries, taking 30+s for me).

It also gets rid of two explicit dependencies (once_cell, url)... but ureq depends on both, so they don't actually disappear from the dependency tree. 🤷

I chose ureq because it doesn't seem to add much compilation time. With some benchmarks on my 4-core laptop:

build type	time before (s)	time after (s)
clean	9-10	10-11
cached, comment change in package/src/utils/build.rs	0.5-0.6	0.5-0.6

There's three commits, which can be reviewed individually. The middle one is the interesting one, with the other two minor related refactors.

[huonw]

I did a CI build without the cache, because the cached build wasn't exercising this new code path. Seems to work ✅