gitlab-ci: Manually set SPHINXBUILD on Windows

For some reason configure seems unable to find it on its own. Let's try
giving it a hint.

Addresses #16398.
22 jobs for !1465 with wip/windows-sphinx in 593 minutes and 44 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Lint
passed #130976
lint
ghc-linters

00:00:15

passed #130977
lint
lint-linters

00:00:16

passed #130980
lint
lint-submods-mr

00:00:15

passed #130978
lint
lint-testsuite

00:00:15

passed #130979
lint
typecheck-testsuite

00:00:15

 
  Build
passed #130982
x86_64-linux
hadrian-ghc-in-ghci

00:17:11

passed #130984
x86_64-linux
release-x86_64-linux-deb9-dwarf

01:38:58

passed #131432
x86_64-linux
validate-x86_64-linux-deb9-debug

03:29:23

passed #130981
x86_64-linux
validate-x86_64-linux-deb9-hadrian

02:12:45

failed #130983
x86_64-linux
validate-x86_64-linux-deb9-debug

02:13:58

 
  Full Build
failed #130986
aarch64-linux allowed to fail
validate-aarch64-linux-deb9

03:17:07

passed #130987
x86_64-linux
validate-i386-linux-deb9

01:49:07

passed #130985
x86_64-darwin
validate-x86_64-darwin

00:56:36

passed #130988
x86_64-linux
validate-x86_64-linux-deb9-integer-simple

01:59:44

passed #130989
x86_64-linux
validate-x86_64-linux-deb9-unreg

03:41:56

passed #130990
x86_64-linux
validate-x86_64-linux-fedora27

01:59:21

failed #130992
x86_64-windows allowed to fail
validate-x86_64-windows

02:31:19

failed #130991
x86_64-windows allowed to fail
validate-x86_64-windows-hadrian

02:42:04

 
  Cleanup
passed #130993
x86_64-darwin
cleanup-darwin

00:00:05

 
  Packaging
passed #130994
x86_64-linux
doc-tarball

00:04:35

 
  Testing
manual #130995
x86_64-linux allowed to fail manual
hackage
passed #130996
x86_64-linux
perf-nofib

00:24:07

 
Name Stage Failure
failed
validate-aarch64-linux-deb9 Full Build
$ cp -Rf $HOME/.cabal cabal-cache
Uploading artifacts...
ghc-*.tar.xz: found 1 matching files
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=130986 responseStatus=201 Created token=Lk122zap
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=130986 responseStatus=201 Created token=Lk122zap
ERROR: Job failed: exit code 1
failed
validate-x86_64-windows Full Build
junit.xml: found 1 matching files                  
WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=130992 responseStatus=201 Created token=9ybrat3w
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=5944 revision=1f513601 version=11.10.1
junit.xml: found 1 matching files

WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=130992 responseStatus=201 Created token=9ybrat3w
ERROR: Job failed: exit status 2
failed
validate-x86_64-windows-hadrian Full Build
junit.xml: found 1 matching files                  
WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=130991 responseStatus=201 Created token=ihREe-MJ
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=4508 revision=1f513601 version=11.10.1
junit.xml: found 1 matching files

WARNING: Failed to load system CertPool:crypto/x509: system root pool is not available on Windows
Uploading artifacts to coordinator... ok
id=130991 responseStatus=201 Created token=ihREe-MJ
ERROR: Job failed: exit status 1