vscode: compute S prefix in Python instead of UNPACKDIR=WORKDIR

The first cut at this fix set UNPACKDIR ??= ${WORKDIR} so kirkstone /
scarthgap (which don't define UNPACKDIR) would still work, but
bitbake.conf on styhead+ rejects the literal 'UNPACKDIR = ${WORKDIR}'
assignment ('Using UNPACKDIR = ${WORKDIR} is not supported').

Compute the prefix via a Python expression instead: use UNPACKDIR if
the release has set one, fall back to WORKDIR otherwise. Same end
result on every supported release without tripping the assertion.

Author: EmbeddedAndroid

recipes-devtools/vscode/vscode_1.120.0.bb

@@ -44,12 +44,13 @@ SRC_URI[vscode-x64.sha256sum]   = "510426eb23d330bf25d84fe88e49a08d965d6b21957b8
 SRC_URI[vscode-arm64.sha256sum] = "69c0d1d0534cd4173e2b3dbee5d001ed5c2bd0c846bb22dca917312b64eb1baf"
 SRC_URI[vscode-armhf.sha256sum] = "1662d3dd08a3602544bc4ca1b091e04d5393a23414607b7c34f0570ebd0d5daa"
 
-# Whinlatter+ split WORKDIR / UNPACKDIR; tarballs land under UNPACKDIR
-# while WORKDIR is now reserved for build artefacts. Older releases
-# (kirkstone..walnascar) didn't have UNPACKDIR at all -- give it a
-# WORKDIR fallback so the recipe works on every supported release.
-UNPACKDIR ??= "${WORKDIR}"
-S = "${UNPACKDIR}/VSCode-linux-${VSCODE_ARCH}"
+# Styhead+ introduced UNPACKDIR as the directory do_unpack writes to,
+# distinct from WORKDIR which is reserved for build artefacts. We have
+# to source S from UNPACKDIR on those releases; on kirkstone/scarthgap
+# UNPACKDIR isn't defined, so fall back to WORKDIR. A literal
+# UNPACKDIR = ${WORKDIR} assignment is rejected by bitbake.conf's QA
+# on styhead+, so compute the prefix via Python instead.
+S = "${@(d.getVar('UNPACKDIR') or d.getVar('WORKDIR'))}/VSCode-linux-${VSCODE_ARCH}"
 
 # These are arch-specific prebuilt ELFs, not noarch content; bin_package
 # inherits allarch which is wrong here. Override back to per-machine