CI: Re-enable backtrace on FreeBSD

Issue #132185 was closed with the following commit:

rust-lang/rust@8d8028f

Author: neuschaefer

.cirrus.yml

@@ -3,9 +3,7 @@ task:
   env:
     HOME: /tmp  # cargo cache needs it
     TARGET: x86_64-unknown-freebsd
-    # FIXME(freebsd): FreeBSD has a segfault when `RUST_BACKTRACE` is set
-    # https://github.com/rust-lang/rust/issues/132185
-    RUST_BACKTRACE: "0"
+    RUST_BACKTRACE: "1"
   matrix:
     - name: nightly freebsd-13 i686
       # Test i686 FreeBSD in 32-bit emulation on a 64-bit host.