python: handle missing /proc/PID/environ in LSF uri resolver

Problem: There's a small race in the LSF uri resolver if a flux-broker
process for the calling user exits between when it is listed by ps(1)
and the resolver attempts to open the /proc/PID/environ file. In this
case, instead of continuing to try other brokers `flux-uri` aborts with

 flux-uri: ERROR: No such file or directory: '/proc/PID/environ'

Catch the FileNotFound exception when trying to read /proc/PID/environ
and treat this the same as if the broker did not match the LSF jobid.

src/bindings/python/flux/uri/resolvers/lsf.py

@@ -48,9 +48,13 @@ def lsf_find_compute_node(jobid):
 
 
 def check_lsf_jobid(pid, jobid):
-    with open(f"/proc/{pid}/environ", encoding="utf-8") as envfile:
-        if f"LSB_JOBID={jobid}" in envfile.read():
-            return True
+    try:
+        with open(f"/proc/{pid}/environ", encoding="utf-8") as envfile:
+            if f"LSB_JOBID={jobid}" in envfile.read():
+                return True
+    except FileNotFoundError:
+        # if pid disappears while we try to read it, this is a False
+        pass
     return False