guix: cache GUI depends separately

The GUI manifest has a distinct Guix profile and consequently produces
different depends build IDs. Previously this mean that the gui build
would invalidate the build cache for the non-guix build.

Move guix builds under a GUIX/ root, and further BUILD/ and GUI/ trees,
so that local developer builds, GUI and non GUI guix builds can all
co-exist together.

There will end up being more copies of build depends packages, (local
builds, and two guix variants), but these all use different toolchains
and toolsets so this is desirable.

contrib/guix/README.md

@@ -151,8 +151,8 @@ If you perform a lot of builds and have a bunch of worktrees, you may find it
 more efficient to keep the depends tree's download cache, build cache, and SDKs
 outside of the worktrees to avoid duplicate downloads and unnecessary builds. To
 help with this situation, the `guix-build` script honours the `SOURCES_PATH`,
-`BASE_CACHE`, and `SDK_PATH` environment variables and will pass them on to the
-depends tree so that you can do something like:
+`BASE_CACHE`, and `SDK_PATH` environment variables so that you can do something
+like:
 
 ```sh
 env SOURCES_PATH="$HOME/depends-SOURCES_PATH" BASE_CACHE="$HOME/depends-BASE_CACHE" SDK_PATH="$HOME/macOS-SDKs" ./contrib/guix/guix-build
@@ -249,9 +249,10 @@ details.
 
 * _**BASE_CACHE**_
 
-  Set the depends tree cache for built packages. This is passed through to the
-  depends tree. Setting this to the same directory across multiple builds of the
-  depends tree can eliminate unnecessary building of packages.
+  Set the root directory for cached built packages. Non-GUI and GUI builds use
+  the `GUIX/BUILD` and `GUIX/GUI` subdirectories, respectively, so their caches do
+  not evict each other's packages. Setting this to the same directory across
+  multiple Guix builds can eliminate unnecessary building of packages.
 
   The path that this environment variable points to **must be a directory**, and
   **NOT a symlink to a directory**.

contrib/guix/libexec/build_linux.sh

@@ -11,6 +11,8 @@ source "$(dirname "${BASH_SOURCE[0]}")/setup.sh"
 # setup gcc toolchain
 gcc_toolchain
 
+BASE_CACHE="${BASE_CACHE:-$PWD/depends/built}/GUIX/BUILD"
+
 # Build the depends tree
 make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \

contrib/guix/libexec/build_linux_gui.sh

@@ -11,6 +11,8 @@ source "$(dirname "${BASH_SOURCE[0]}")/setup.sh"
 # setup gcc toolchain
 gcc_toolchain
 
+BASE_CACHE="${BASE_CACHE:-$PWD/depends/built}/GUIX/GUI"
+
 # Build the depends tree
 make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \

contrib/guix/libexec/build_macos.sh

@@ -11,6 +11,8 @@ source "$(dirname "${BASH_SOURCE[0]}")/setup.sh"
 # Setup toolchain
 llvm_toolchain
 
+BASE_CACHE="${BASE_CACHE:-$PWD/depends/built}/GUIX/BUILD"
+
 # Build the depends tree
 make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \

contrib/guix/libexec/build_macos_gui.sh

@@ -11,6 +11,8 @@ source "$(dirname "${BASH_SOURCE[0]}")/setup.sh"
 # Setup toolchain
 llvm_toolchain
 
+BASE_CACHE="${BASE_CACHE:-$PWD/depends/built}/GUIX/GUI"
+
 # Build the depends tree
 make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \

contrib/guix/libexec/build_win.sh

@@ -11,6 +11,8 @@ source "$(dirname "${BASH_SOURCE[0]}")/setup.sh"
 # setup mingw-w64 toolchain
 mingw_w64_toolchain
 
+BASE_CACHE="${BASE_CACHE:-$PWD/depends/built}/GUIX/BUILD"
+
 # Build the depends tree
 make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \

contrib/guix/libexec/build_win_gui.sh

@@ -11,6 +11,8 @@ source "$(dirname "${BASH_SOURCE[0]}")/setup.sh"
 # setup mingw-w64 toolchain
 mingw_w64_toolchain
 
+BASE_CACHE="${BASE_CACHE:-$PWD/depends/built}/GUIX/GUI"
+
 # Build the depends tree
 make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    ${V:+V=1} \