tasks.py: add option to install libh2o manually

[romeroalx]

Short description

This PR modifies tasks.py adding the option to install libh2o "manually" when it is not available via APT, as is the case for Debian Trixie.

Required for: https://github.com/PowerDNS/base-pdns-ci-image/issues/20

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [x] read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• [ ] compiled this code
• [x] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 20103290699

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 51 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+4.4%) to 73.335%

---

Files with Coverage Reduction	New Missed Lines	%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.8%
modules/pipebackend/pipebackend.cc	2	60.79%
pdns/recursordist/aggressive_nsec.cc	2	66.39%
modules/gpgsqlbackend/spgsql.cc	3	67.94%
pdns/iputils.cc	3	58.81%
pdns/recursordist/test-syncres_cc2.cc	3	89.12%
pdns/recursordist/rec-taskqueue.cc	4	40.14%
pdns/misc.cc	6	61.34%
pdns/dnsdistdist/dnsdist-carbon.cc	9	62.01%
pdns/dnsdistdist/dnsdist-tcp.cc	9	77.3%
<!--	Total:	51

Totals
Change from base Build 20093099049:	4.4%
Covered Lines:	128792
Relevant Lines:	164859

---

💛 - Coveralls

[rgacogne]

Thanks!