ci(windows): export the Rust env for the step that checks it

The detection branch found the toolchain, wrote RUSTUP_HOME and CARGO_HOME to
$GITHUB_ENV, and then verified with `rustc --version` in the same step.
$GITHUB_ENV only reaches later steps, so that verification ran with neither
variable set and the rustup shim fell back to %USERPROFILE%\.rustup. On this
runner that path is a file, so it failed with os error 183 while a perfectly
good toolchain sat at C:\Rust.

The step's own comment already explained why the shims need RUSTUP_HOME. It
just never exported it for its own use. The install branch does export before
its check, which is why only the detection path broke, and why it broke on a
runner that already had Rust rather than on a fresh one.

No app code changes, so hash-bags-ios-1.0.3 stays where it is: Android, Linux
and iOS are building from that commit and the artifacts are unaffected.

.github/workflows/build-windows.yml

@@ -101,8 +101,17 @@ jobs:
               # LocalSystem service account.
               if [[ -d "$(dirname "$(dirname "$d")")/rustup" ]]; then
                 RUSTUP_DIR="$(dirname "$(dirname "$d")")/rustup"
-                echo "RUSTUP_HOME=$(cygpath -w "$RUSTUP_DIR")" >> "$GITHUB_ENV"
-                echo "CARGO_HOME=$(cygpath -w "$(dirname "$d")")"  >> "$GITHUB_ENV"
+                # Export for THIS step as well as writing to $GITHUB_ENV.
+                # $GITHUB_ENV only reaches later steps, and the verification on
+                # the next line is itself a rustup shim: without the export it
+                # falls back to %USERPROFILE%\.rustup, which on this runner is
+                # a file rather than a directory, so it fails with os error 183
+                # having ignored the toolchain we just found.
+                RUSTUP_HOME="$(cygpath -w "$RUSTUP_DIR")"
+                CARGO_HOME="$(cygpath -w "$(dirname "$d")")"
+                export RUSTUP_HOME CARGO_HOME
+                echo "RUSTUP_HOME=$RUSTUP_HOME" >> "$GITHUB_ENV"
+                echo "CARGO_HOME=$CARGO_HOME"   >> "$GITHUB_ENV"
               fi
               rustc --version
               exit 0