CI is broken again

[eldruin]

This leads to different versions of the books when read on different domains: docs.rust-embedded.org vs rust-embedded.github.io.
I do not know what the cause is. See last CI log:

~/build/rust-embedded/docs
+rustdoc --markdown-css rust.css --markdown-no-toc index.md
+rustdoc --markdown-css rust.css faq.md
+books=(book discovery embedonomicon)
+local books
++mktemp -d
+local tmpdir=/tmp/tmp.cfMiduW7yS
+for book in '"${books[@]}"'
+git clone https://github.com/rust-embedded/book /tmp/tmp.cfMiduW7yS/book
Cloning into '/tmp/tmp.cfMiduW7yS/book'...
+cd /tmp/tmp.cfMiduW7yS/book
+mdbook build
2021-02-08 12:17:06 [INFO] (mdbook::book): Book building has started
2021-02-08 12:17:06 [INFO] (mdbook::book): Running the html backend
+mv /tmp/tmp.cfMiduW7yS/book/book doc/book
+for book in '"${books[@]}"'
+git clone https://github.com/rust-embedded/discovery /tmp/tmp.cfMiduW7yS/discovery
Cloning into '/tmp/tmp.cfMiduW7yS/discovery'...
+cd /tmp/tmp.cfMiduW7yS/discovery
+mdbook build
2021-02-08 12:17:07 [INFO] (mdbook::book): Book building has started
2021-02-08 12:17:07 [INFO] (mdbook::book): Running the html backend
2021-02-08 12:17:07 [ERROR] (mdbook::utils): Error: Rendering failed
2021-02-08 12:17:07 [ERROR] (mdbook::utils): 	Caused By: Failed to read "/tmp/tmp.cfMiduW7yS/discovery/src/08-leds-again/target"
2021-02-08 12:17:07 [ERROR] (mdbook::utils): 	Caused By: No such file or directory (os error 2)

[eldruin]

This was fixed in #33