riscv-64-cross-compile fails non-deterministically: race in build rules?

Happened once and then did not repeat:
https://github.com/DynamoRIO/dynamorio/actions/runs/4663833868/jobs/8255522120?pr=5967
```
[ 75%] Building C object ext/drmgr/CMakeFiles/drmgr_static.dir/drmgr.c.o
In file included from /home/runner/work/dynamorio/dynamorio/build_riscv64-release-external/cmake/../include/dr_ir_macros.h:53,
from /home/runner/work/dynamorio/dynamorio/build_riscv64-release-external/cmake/../include/dr_api.h:96,
from /home/runner/work/dynamorio/dynamorio/ext/drmgr/drmgr.c:37:
/home/runner/work/dynamorio/dynamorio/ext/drmgr/drmgr.c: In function ‘drmgr_insert_read_tls_field’:
/home/runner/work/dynamorio/dynamorio/build_riscv64-release-external/cmake/../include/dr_ir_macros_riscv64.h:59:64: error: ‘OP_c_nop’ undeclared (first use in this function)
59 | #define XINST_CREATE_load(dc, r, m) instr_create_1dst_1src(dc, OP_c_nop, r, m)
| ^~~~~~~~
/home/runner/work/dynamorio/dynamorio/ext/drmgr/drmgr.c:2759:9: note: in expansion of macro ‘XINST_CREATE_load’
2759 | XINST_CREATE_load(
| ^~~~~~~~~~~~~~~~~
```