fix dump problem (#35673)

if dump file existed it was not truncating the file, resulting in
a file with unaltered filesize, with the new content at the beginning,
"padded" with the tail of the old content, since the new content was
not enough to overwrite it.

lib/spack/spack/util/environment.py

@@ -138,7 +138,7 @@ def dump_environment(path, environment=None):
     use_env = environment or os.environ
     hidden_vars = set(["PS1", "PWD", "OLDPWD", "TERM_SESSION_ID"])
 
-    fd = os.open(path, os.O_WRONLY | os.O_CREAT, 0o600)
+    fd = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
     with os.fdopen(fd, "w") as env_file:
         for var, val in sorted(use_env.items()):
             env_file.write(