ccp4 README: document explicit --tmpdir/--distdir build command

The ~/.Pmodules/Pmodules.yaml route did not reliably set download_dir,
so the maintained recipe now passes both dirs on the command line;
readers must adapt the p-group path to their own.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ccp4/README.md

@@ -17,27 +17,31 @@ macromolecular crystallography. https://www.ccp4.ac.uk/
 
 ## Build on Ra
 
-One-time per user: put the Pmodules build tmp and download cache on
-/das instead of /var/tmp and $HOME (the tarball alone would blow the
-home quota):
-
-```bash
-mkdir -p /das/work/p21/p21515/.cache/Pmodules/distfiles ~/.Pmodules
-cat > ~/.Pmodules/Pmodules.yaml <<'EOF'
-tmp_dir: /das/work/p21/p21515/.cache/Pmodules
-download_dir: /das/work/p21/p21515/.cache/Pmodules/distfiles
-EOF
-```
-
-Then:
+Build tmp and download cache MUST be on /das, not the defaults
+(/var/tmp is too small, the tarball alone blows the $HOME quota — a
+download dying with "Failure writing output to destination" means
+exactly that). Pass both dirs explicitly on the command line so there
+is no doubt where things land — **adapt the p-group path
+(`p21/p21515`) to your own**:
 
 ```bash
 module use unstable && module load modbuild/2.1.3
 cd ccp4
-modbuild build --prep 9.0   # download/cache only; also verifies tmp_dir took effect
-modbuild build 9.0
+mkdir -p /das/work/p21/p21515/.cache/Pmodules
+modbuild build \
+    --tmpdir=/das/work/p21/p21515/.cache/Pmodules \
+    --distdir=/das/work/p21/p21515/.cache/Pmodules \
+    --clean-install -v --debug 9.0
 ```
 
+(`--clean-install` removes a previous broken install first; `-v
+--debug` make the otherwise silent long steps visible.)
+
+Alternative: `~/.Pmodules/Pmodules.yaml` with top-level `tmp_dir:` and
+`download_dir:` keys sets the same defaults per user — but in practice
+`download_dir` did not reliably take effect there, so the explicit
+flags above are the maintained route.
+
 If the download fails (expired sid), fetch a fresh link from
 https://www.ccp4.ac.uk/download/ and update `files/config.yaml`, or drop a
 hand-downloaded tarball into the distfiles dir under the `name:` given in
@@ -65,12 +69,12 @@ Growing `du` = working. Only investigate if it's frozen for 10+ min.
 - **"done" but broken install**: modbuild has no errexit in hooks — errors in
   `pbuild::install` used to scroll by and the build still reported done. The
   build script now `std::die`s on every critical step.
-- **Interrupted download**: prep now detects a truncated tarball with
-  `gzip -t` and resumes it with `wget --continue` — just re-run
-  `modbuild build 9.0`. A download that dies with
+- **Interrupted download**: prep detects a truncated tarball with
+  `gzip -t` and resumes it with `wget --continue` — just re-run the
+  build command above. A download that dies with
   `Failure writing output to destination` means quota/disk full at the
-  distfiles dir, not a network problem — free space or move `download_dir:`
-  (see above) before retrying.
+  distfiles dir, not a network problem — check `--distdir` points to
+  /das before retrying.
 - The tarball unpacks to `ccp4-9` (not `ccp4-9.0` as the CCP4 install doc
   implies); the build globs `ccp4-*/` instead of assuming the name.