chore(github): reduce compile time in build, nix, and release workflows

Summary: When building in CI, we just build everything from scratch in the `dev`
profile, so just turn off debuginfo and incremental compilation data. These
aren't reused, but more importantly take up more space and CPU time to generate;
that's more expensive on slower machines and when using The Cloud(TM) to build.

Together, these take a fresh build (`rm -rf target/` ahead of time) from about
40s to 30s on my machine.

Signed-off-by: Austin Seipp <[e-mail]>
Change-Id: Id6e706634dbaf88f6ccd7f457409ad7c

.github/workflows/build.yml

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 permissions: read-all
 
+env:
+  CARGO_INCREMENTAL: 0
+  CARGO_PROFILE_DEV_DEBUG: 0
+
 jobs:
   build:
     strategy:

.github/workflows/release.yml

···
 
 permissions: read-all
 
+env:
+  CARGO_INCREMENTAL: 0
+
 jobs:
   build-release:
     name: build-release

flake.nix

···
           ZSTD_SYS_USE_PKG_CONFIG = "1";
           LIBSSH2_SYS_USE_PKG_CONFIG = "1";
           NIX_JJ_GIT_HASH = self.rev or "";
+          CARGO_INCREMENTAL = "0";
           postInstall = ''
             $out/bin/jj util mangen > ./jj.1
             installManPage ./jj.1