It would be useful for executables to be able to tell whether they are running within a Please build environment, whether or not that environment is sandboxed. One definitive use case is when deciding whether or not resources should be loaded from $TMP_DIR, which has a special (and well-defined) meaning within a Please build environment but not outside of one, even though the variable may still be defined in the environment. The java-rules and python-rules plugins would both benefit from this, given that java_binary JARs and python_binary pexes have runtimes that are expected to be discovered dynamically but will be located at different paths depending on whether they are executing under plz test (inside a build environment) or plz run (outside a build environment).

Define the PLZ_ENV variable in Please build environments. This of course does not guarantee that the environment was created by Please, but is a better indicator of that than heuristics such as testing for the presence of other variables that happen to be defined in certain build environments, such as BUILD_CONFIG and _TEST_ID.