Revert "Remove custom valgrind suppressions"

This reverts commit ad96472.

cargo-valgrind ships outdated suppressions [1], and it looks like the
situation won't improve anytime soon, so bring custom suppressions back.

[1]: jfrimmel/cargo-valgrind#126

Author: purplesyringa

.github/workflows/ci.yml

@@ -366,6 +366,8 @@ jobs:
   valgrind:
     timeout-minutes: 5
     runs-on: ubuntu-latest
+    env:
+      VALGRINDFLAGS: --suppressions=valgrind.supp
     if: success() || failure()
     steps:
       - name: Checkout

valgrind.supp

@@ -0,0 +1,9 @@
+{
+   <std_thread_current>
+   Memcheck:Leak
+   match-leak-kinds: possible
+   fun:malloc
+   ...
+   fun:_ZN3std6thread6Thread3new*
+   fun:_ZN3std6thread7current12init_current*
+}