Cherry-pick #8849 to 6.5: Fix inherits environment variables (#8856)

Cherry-pick of PR #8849 to 6.5 branch. Original message: 

We were inheriting environment variables on windows only, this make sure
the variables are inherited on unix too. Because the system module in
metricbeat uses `lsof` since the environment variables were cleared the
$PATH was not available so the binary was not found.

fixes: #8848

libbeat/tests/system/beat/beat.py

@@ -45,12 +45,11 @@ def __init__(self, args, outputfile, env={}):
         self.env = env
 
     def start(self):
+        # ensure that the environment is inherited to the subprocess.
+        variables = os.environ.copy()
+        variables = variables.update(self.env)
 
         if sys.platform.startswith("win"):
-            # ensure that the environment is inherited to the subprocess.
-            variables = os.environ.copy()
-            variables = variables.update(self.env)
-
             self.proc = subprocess.Popen(
                 self.args,
                 stdin=self.stdin_read,
@@ -66,7 +65,7 @@ def start(self):
                 stdout=self.output,
                 stderr=subprocess.STDOUT,
                 bufsize=0,
-                env=self.env)
+                env=variables)
             # If a "No such file or directory" error points you here, run
             # "make metricbeat.test" on metricbeat folder
         return self.proc