feat(tvix/store): use reqwests' rustls-native-roots feature

This makes reqwest honor `SSL_CERT_FILE` - previously it was using the
chain bundled in webpki-roots.

`object_store` pulls in `reqwest` with this feature, and the cargo
solver will enable that feature globally as soon as we pull it in, as it
assumes features are additive.

This requires setting `SSL_CERT_FILE` when running tests, otherwise
they'll fail with the unhelpful "NotFound" error.

This was quite some fun to debug, why adding `object_store` to
tvix-castore suddenly made tvix-store tests fail!

Change-Id: I64fc82b4d994715480efdb1ffecb279716456ab9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11090
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/Cargo.lock

@@ -2233,6 +2233,7 @@ dependencies = [
  "percent-encoding",
  "pin-project-lite",
  "rustls",
+ "rustls-native-certs",
  "rustls-pemfile",
  "serde",
  "serde_json",
@@ -2247,7 +2248,6 @@ dependencies = [
  "wasm-bindgen-futures",
  "wasm-streams",
  "web-sys",
- "webpki-roots",
  "winreg",
 ]
 
@@ -3802,12 +3802,6 @@ dependencies = [
  "wasm-bindgen",
 ]
 
-[[package]]
-name = "webpki-roots"
-version = "0.25.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1778a42e8b3b90bff8d0f5032bf22250792889a5cdc752aa0020c84abe3aaf10"
-
 [[package]]
 name = "which"
 version = "4.4.2"