Remove uses of `set_var`

[RalfJung]

set_var has a problem and there are plans to make it unsafe. So we should find replacements for our uses of it.

• miri/src/bin/miri.rs

  Line 169 in d0fd429

  env::set_var("RUSTC_LOG_ENTRY_EXIT", "1");

  
  Needs some way to pass this flag to the rustc logging machinery without an env var.

• miri/src/bin/miri.rs

  Lines 191 to 198 in d0fd429

  	if log::Level::from_str(&var).is_ok() {
  	env::set_var(
  	"RUSTC_LOG",
  	format!("rustc_middle::mir::interpret={var},rustc_const_eval::interpret={var}"),
  	);
  	} else {
  	env::set_var("RUSTC_LOG", &var);
  	}

  
  Needs a way to init rustc logging by giving it the value of the filter expression we want, instead of the name of an env var with the filter.

• miri/cargo-miri/src/setup.rs

  Line 121 in d0fd429

  std::env::set_var("MIRI_SYSROOT", &sysroot_dir);

  
  This one we could already do: find some other way to propagate this information to all parts of the binary, and to all created sub-processes.

[RalfJung]

#3006 removes the one in cargo-miri.

The others need logging machinery extensions first, so this is blocked on there being other APIs to control rustc logging.

[RalfJung]

All uses of set_var in Miri itself are gone now thanks to rust-lang/rust#117814. All we have left is some uses in compiletest:

miri/tests/compiletest.rs

Lines 235 to 238 in d4d85a8

	// Add a test env var to do environment communication tests.
	env::set_var("MIRI_ENV_VAR_TEST", "0");
	// Let the tests know where to store temp files (they might run for a different target, which can make this hard to find).
	env::set_var("MIRI_TEMP", env::temp_dir());