Run examples in parallel

[WaffleLapkin]

This changes the time for me locally 1.69 secs -> 0.41 secs. Probably doesn't matter for CI, but nice for local dev.

[WaffleLapkin]

diff --git a/code/examples/stderr/unsafe_3_1.stderr b/code/examples/stderr/unsafe_3_1.stderr
index e69de29..2ff2c57 100644
--- a/code/examples/stderr/unsafe_3_1.stderr
+++ b/code/examples/stderr/unsafe_3_1.stderr
@@ -0,0 +1,9 @@
+error[E0463]: can't find crate for `std`
+  |
+  = note: the `x86_64-unknown-linux-gnu` target may not be installed
+  = help: consider downloading the target with `rustup target add x86_64-unknown-linux-gnu`
+  = help: consider building the standard library from source with `cargo build -Zbuild-std`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0463`.

???

[Nilstrieb]

uh, this PR was quite broken wrt toolchains, I made a mess in builder sorry :3

[Nilstrieb]

It now changes the environment variable and CWD of the builder process itself to ensure that everything works and that forgetting to change it on a command (like you did) won't lead to weird bugs.