When performing Guix builds for {x86_64,arm64}-apple-darwin hosts across different commits, all packages in depends are rebuilt even if there are no changes in either the depends or contrib/guix subdirectories.
This occurs because the SOURCE_DATE_EPOCH environment variable enables Clang’s -source-date-epoch option, which then appears in the output of clang -v -E -xc -o /dev/null - < /dev/null. For example:
0$ SOURCE_DATE_EPOCH=1767855465 clang -v -E -xc++ -o /dev/null - < /dev/null
1clang version 21.1.7 (Fedora 21.1.7-1.fc43)
2Target: x86_64-redhat-linux-gnu
3Thread model: posix
4InstalledDir: /usr/bin
5Configuration file: /etc/clang/x86_64-redhat-linux-gnu-clang.cfg
6System configuration file directory: /etc/clang/
7Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/14
8Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/15
9Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/15
10Candidate multilib: .;@m64
11Candidate multilib: 32;@m32
12Selected multilib: .;@m64
13 (in-process)
14 "/usr/bin/clang-21" -cc1 -triple x86_64-redhat-linux-gnu -E -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name - -mrelocation-model static -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/hebasto -v -fcoverage-compilation-dir=/home/hebasto -resource-dir /usr/bin/../lib/clang/21 -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15 -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/x86_64-redhat-linux -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/backward -internal-isystem /usr/bin/../lib/clang/21/include -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../x86_64-redhat-linux/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -source-date-epoch 1767855465 -fdeprecated-macro -ferror-limit 19 -fmessage-length=180 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /dev/null -x c++ -
15clang -cc1 version 21.1.7 based upon LLVM 21.1.7 default target x86_64-redhat-linux-gnu
16ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../x86_64-redhat-linux/include"
17ignoring nonexistent directory "/include"
18#include "..." search starts here:
19#include <...> search starts here:
20 /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15
21 /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/x86_64-redhat-linux
22 /usr/bin/../lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/backward
23 /usr/bin/../lib/clang/21/include
24 /usr/local/include
25 /usr/include
26End of search list.
As a result, each package id differs between builds, which causes the script to treat the toolchain as changed and triggers unnecessary rebuilds.
This PR resolves this issue by unsetting SOURCE_DATE_EPOCH in a subshell before collecting toolchain details.
