ci: allow unstable features in some PGO benchmarks

Author: pietroalbini

src/ci/pgo.sh

@@ -7,7 +7,7 @@ rm -rf /tmp/rustc-pgo
 python2.7 ../x.py build --target=$PGO_HOST --host=$PGO_HOST \
     --stage 2 library/std --rust-profile-generate=/tmp/rustc-pgo
 
-./build/$PGO_HOST/stage2/bin/rustc --edition=2018 \
+RUSTC_BOOTSTRAP=1 ./build/$PGO_HOST/stage2/bin/rustc --edition=2018 \
     --crate-type=lib ../library/core/src/lib.rs
 
 # Download and build a single-file stress test benchmark on perf.rust-lang.org.
@@ -16,7 +16,9 @@ function pgo_perf_benchmark {
     local github_prefix=https://raw.githubusercontent.com/rust-lang/rustc-perf/$PERF
     local name=$1
     curl -o /tmp/$name.rs $github_prefix/collector/benchmarks/$name/src/lib.rs
-    ./build/$PGO_HOST/stage2/bin/rustc --edition=2018 --crate-type=lib /tmp/$name.rs
+
+    RUSTC_BOOTSTRAP=1 ./build/$PGO_HOST/stage2/bin/rustc --edition=2018 \
+        --crate-type=lib /tmp/$name.rs
 }
 
 pgo_perf_benchmark externs