reclient doesn't cache any thing if -exec_root is set to any path other than root directory "/"

I'm invetigating why the cache hit rates is low on one of my project, and it seems like the source path may impact, as my contianer provider assigns me a random path everytime I apply for the build resources like /path/to/xxx, where xxx changes everytime.

Then to confirm this, I setup a local environment like below steps:
1. used opencv as example project, https://github.com/opencv/opencv
2. cloned the source code and placed it to /home/myname/ws/opencv
3. setup reclient environment. Configuration pasted at the end of this post.
4. generated make files: `cmake /home/myname/ws/opencv -DCMAKE_C_COMPILER_LAUNCHER=/path/to/reclient/rewrapper;-cfg=/path/to/reconfig;-exec_root=/ -DCMAKE_CXX_COMPILER_LAUNCHER=/path/to/reclient/rewrapper;-cfg=/path/to/reconfig;-exec_root=/ -B /home/myname/ws/opencv/mybuild`
5. Built opencv for the first time `make -j$(nproc)`, got `RBE Stats: down 3.15 MB, up 0 B, 174 cache hits, 1428 local executions`
6. Built opencv for the second time, got `RBE Stats: down 379.81 MB, up 217.31 KB, 1601 cache hits, 1 local execution` , which shown 100% cache hits
7. To invesitgate if random path impacts, I copied the whole source code from opencv folder to another folder /home/myname/ws/opencv2 and built, the result indicates the cache is not hit: `RBE Stats: down 3.15 MB, up 0 B, 174 cache hits, 1428 local executions`
8. To investigate if -exec_root impacts, I set different paths to exec_path. Set `-exec_root=/home/myname/ws/opencv`, no matter how many times I built the project it always executes locally , hit rates: `RBE Stats: down 3.15 MB, up 0 B, 174 cache hits, 1428 local executions`

Questions:
1. It seems that the path of source code impacts cache, I guess it is probably because cmake generates makefiles using absolute paths. How to address it if my guess is ture?
2. Why exec_root is set to the root(`-exec_root=/home/myname/ws/opencv` of the source code doesn't work as described in the document? But set to root of the file systems(`-exec_root=/`) works, why?

**Configuration**
```
service=remotecache:9980
instance=fuse
server_address=unix:///tmp/reproxy.sock
log_dir=/tmp/reclient
output_dir=/tmp/reclient/output
proxy_log_dir=/tmp/reclient/proxy
depsscanner_address=exec:///reclient/scandeps_server
service_no_auth=true
service_no_security=true
use_rpc_credentials=false
use_application_default_credentials=false
canonicalize_working_dir=false
exec_strategy=local
env_var_allowlist=INCLUDE,LIB,LIBPATH
labels=type=compile,lang=cpp,compiler=clang
reclient_timeout=30s
dump_input_tree=true
```

Reproduce the OpenCV builds described in the issue, comparing -exec_root=/ with -exec_root=/home/myname/ws/opencv and builds from /home/myname/ws/opencv2. Start by tracing reclient's exec_root and cache-key handling; done means identifying why the non-root setting misses the cache and either correcting that behavior or documenting the limitation.

Written by the indexing model from the issue text.