Switch to ureq for our simple sync HTTP get request (#1167)

Turns out that you aren't allowed to call `reqwest::blocking::get()`
from inside `tokio::block_on()`

reqwest uses tokio internally, which means that
`reqwest::blocking::get()` tries to use tokio blocking APIs, but when
you're already inside a tokio runtime (which we are due to our top level
`tokio::block_on()`) you can't use any additional tokio blocking APIs.
It results in a full panic and a confusing message.

You can supposedly do stuff like this to push the blocking work onto an
actual blocking thread used by tokio

```
let resp = tokio::task::spawn_blocking(|| {
    cache::get("dplyr")
}).await.unwrap();
```

but thats so awkward to think about, especially considering that we want
the cache to be "warm" most of the time, not requiring tokio at all.

---

Instead, I think we should switch to ureq, which is one of the other
popular but more minimal HTTP clients and is _always blocking_, which is
fine for us right now.

This still requires the tweaks to the Windows github action. ureq uses
ring under the hood (at least for now, see
algesten/ureq#1141). On ARM Windows where we
have an MSVC target of `aarch64-pc-windows-msvc`, ureq needs to compile
in a lot of C code with clang, but ends up finding the Rtools MinGW
tooling that is put on the PATH, and that causes it to fail with an
`<assert.h>` missing header error.

Author: DavisVaughan

.github/workflows/test-windows.yml

@@ -39,11 +39,10 @@ jobs:
           r-version: ${{ matrix.config.r }}
           use-public-rspm: true
           # Putting Rtools related tooling on the PATH causes Rust crate
-          # compilation to pick up Rtools's `link.exe` and mingw32 related
-          # tooling, in particular it can cause aws-lc-sys compilation to fail
-          # on ARM Windows, where it uses clang-cl and has to compile in a bunch
-          # of C code.
-          # https://github.com/aws/aws-lc-rs/blob/main/book/src/requirements/windows.md
+          # compilation to pick up Rtools's MinGW tooling, in particular it can
+          # cause ring (used by ureq) compilation to fail on ARM Windows, where
+          # it uses clang and has to compile in a bunch of C code.
+          # https://github.com/briansmith/ring/blob/main/BUILDING.md
           windows-path-include-rtools: false
 
       - name: Install R Packages Required For Tests