Pin indexmap to 1.8.2 for MSRV compile tasks (#684)

Author: kmahar

.evergreen/MSRV-Cargo.lock

@@ -5,6 +5,11 @@ set -o errexit
 source ./.evergreen/configure-rust.sh
 rustup update $RUST_VERSION
 
+# pin dependencies who have bumped their MSRVs to > ours in recent releases.
+if [  "$MSRV" = "true" ]; then
+    cp .evergreen/MSRV-Cargo.lock Cargo.lock
+fi
+
 if [ "$ASYNC_RUNTIME" = "tokio" ]; then
     FEATURE_FLAGS=snappy-compression,zlib-compression
 
@@ -17,12 +22,6 @@ if [ "$ASYNC_RUNTIME" = "tokio" ]; then
     rustup run $RUST_VERSION cargo build --features tokio-sync,$FEATURE_FLAGS
 
 elif [ "$ASYNC_RUNTIME" = "async-std" ]; then
-    # v2.1 of async-global-executor bumped its MSRV to 1.59, so we need a Cargo.lock
-    # pinning to v2.0 to build with our MSRV.
-    if [  "$MSRV" = "true" ]; then
-        cp .evergreen/MSRV-Cargo.lock Cargo.lock
-    fi
-
     rustup run $RUST_VERSION cargo build --no-default-features --features async-std-runtime
     rustup run $RUST_VERSION cargo build --no-default-features --features sync
 else