fix: pgflex - propagate environment to flex subprocess

Python's subprocess.run docs say that if the env argument is not None,
it will be used "instead of the default behavior of inheriting the
current process’ environment".

However, the environment should be preserved, only adding FLEX_TMP_DIR
to it. For more info, see the following thread:

https://www.postgresql.org/message-id/ot4w2y6es446gjgvhbpp45qt3wx65n5epjxgsu3ghgr63yuizn%40qeurkcofswmv

Author: jjmaestro

src/tools/pgflex

@@ -48,7 +48,7 @@ os.chdir(args.privatedir)
 # contents. Set FLEX_TMP_DIR to the target private directory to avoid
 # that. That environment variable isn't consulted on other platforms, so we
 # don't even need to make this conditional.
-env = {'FLEX_TMP_DIR': args.privatedir}
+os.environ['FLEX_TMP_DIR'] = args.privatedir
 
 # build flex invocation
 command = [args.flex, '-o', args.output_file]
@@ -58,7 +58,7 @@ command += args.flex_flags
 command += [args.input_file]
 
 # create .c file from .l file
-sp = subprocess.run(command, env=env)
+sp = subprocess.run(command)
 if sp.returncode != 0:
     sys.exit(sp.returncode)