remap crates fetched via sparse registry as well (#1888)

on Rust nightlies since about 2023-03-09:
    https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-168-2023-03-09
it seems the default (-on-nightly?) Cargo protocol switched to "sparse".
this has a side effect of caching fetched crates at a different path, so
the `github.com` path remapping is no longer sufficient to normalize all
crate paths.

we then picked that change up back at v1.0.14, bumping nightly forward
a year and change:
    24bdff6#diff-2b1bde2cf3a858b7bf7424cb8bcbf01f35b94dc80b925d9432cbab3319ca9b4e
so local builds of hubris between then and v1.0.26 have source paths
that are not actually remapped :(

thankfully, to make it sufficient we only have to handle the new
`index.crates.io-6f17d22bba15001f` path as well. this in fact is where
*all* cargo-fetched crates.io dependencies end up now.

technically we could forget about `github.com-1ecc6299db9ec823` at this
point, but by keeping it we'll still be correct in the unlikely event
someone were to flip away from `sparse` to `git` for the Cargo protocol.

when i built `gimlet-v1.0.26` with this patch the resulting images no
longer have my home directory in them.

... i am deeply perplexed that with this change the resulting `gimlet-b`
image is 0x2200 bytes larger though???

a funny consequence of this is that to reproduce the hubris releases
from 1.0.14 to ... now ... you actually have to remap the
index.crates.io directory to
`/home/runner/.cargo/registry/src/index.crates.io-6f16d22bba15001f`. by
doing this i just built a `gimlet-b` that matches the one
[here](https://github.com/oxidecomputer/hubris/releases/tag/gimlet-v1.0.26).

Author: iximeow

build/xtask/src/dist.rs

@@ -167,7 +167,7 @@ impl PackageConfig {
             remap_paths.insert(cargo_git, "/git");
 
             // This hash is canonical-ish: Cargo tries hard not to change it
-            // https://github.com/rust-lang/cargo/blob/master/src/cargo/core/source/source_id.rs#L607-L630
+            // https://github.com/rust-lang/cargo/blob/5dfdd59/src/cargo/core/source_id.rs#L755-L794
             //
             // It depends on system architecture, so this won't work on (for example)
             // a Raspberry Pi, but the only downside is that panic messages will
@@ -177,6 +177,15 @@ impl PackageConfig {
                 .join("src")
                 .join("github.com-1ecc6299db9ec823");
             remap_paths.insert(cargo_registry, "/crates.io");
+            // If Cargo uses the sparse registry (stabilized since ~1.72) it caches fetched crates
+            // in a slightly different path. Remap that one as well.
+            //
+            // This path has the same canonical-ish properties as above.
+            let cargo_sparse_registry = cargo_home
+                .join("registry")
+                .join("src")
+                .join("index.crates.io-6f17d22bba15001f");
+            remap_paths.insert(cargo_sparse_registry, "/crates.io");
         }
 
         if let Ok(dir) = std::env::var("CARGO_MANIFEST_DIR") {